Help. Chicken with huge swollen red butt

OK...Something's not right.
My Marans has a huge... fist sized... red swollen lump below her vent on her bottom. It is soft and squishy. I initially thought she might be eggbound, I've never had an eggbound hen & have never seen what one looks like. The swelling is below the vent. I've heard that eggs can back up & swelling can appear. But does it appear below the vent? The anatomy book says this is the area of the small intestine. I have checked her vent and there are no signs of an egg either broken or intact.
I have soaked her in a warm bath for two hours. Lubed her up with olive oil from both ends. She pooped the biggest poo I've ever seen & the swelling seemed to go down a little...But at this point still no egg. Can she poo if she's eggbound?
As of now, I'm about 48 hours into this so according to the books if she had egg perotinitis she'd already be dead.
Could she just be constipated? I mean really, really, constipated?
What should I do next.
I have some H2O soluable tylan...does anyone know what dosage I should use. The instructions are for sheep & pigs. Is this the right stuff? I don't normally use antibiotics but think that this looks like something nasty & internal.
Ideas anyone?

A prolapse maybe? Hope you find out.

Nothing is coming out of the vent so I don't think it is a prolapse.
I don't think she is eggbound because I can't find any evidence of an egg in her vent.
I just read on a site that internal layers can have swollen abdomens.
Also, that egg peritonitis will not kill a hen until an infection becomes systemic. So it can go on for weeks. Maybe this is the problem?
Does anyone have any experience with this.
Am I overreacting? Could she just be stopped up?
Anyone?

Ok, is this what it looks like?

Yes....Any advice?
Otherwise she's happy & eating well etc.
Does she just have a weird looking bald bottom? Or do I need to be concerned?

I have exactly the same problem with one of my two year old hens. What happened to your hen? Did you treat her? Do you know what this problem is? I'm looking for any advice on what to do.

My hen is the same...what ended up happening with your hen?

It could be ascitis (filling with watery fluid). Some cases are caused by serious health issues like liver failure but I have a hen who started filling up with clear water at about a year and half old. I use a needle and syringe and drain her when she does. I used to have to drain her about every three months but then she stops for awhile and then several months may go by. All the while, she is perfectly healthy in all other respects and lays an egg daily. She's so used to getting drained that if I put some food in front of her she will eat while I drain her. I've found it's best to use a larger guage needle and as big a syringe as possible because that way you don't have to stick her as much and you can even unscrew the syringe and the needle can leak on its own.

The first time I drained her, she was so big and so bad off I thought I was going to have to put her down. She was so huge she could no longer walk and I didn't know what to do for her. As a last resort I tried to stick a needle in the huge water balloon hanging below her vent and I got out over a cup of clear water and since I had to stick her so many times, she continued leaking for a while. I placed her on a stack of folded towels and she soaked through all of them. She got right up after that and has been fine ever since. I keep an eye on her because about every three to four months she starts to swell again and I drain her before she gets too big.

Just insert needle into the water balloon that is hanging down. There's no vital organs there and then pull back on the syringe and see what comes out. If it's clear or mostly tinted water, she might be okay with just being drained. If it's thick and can't be drawn out or yolky or bloody, there may be other issues but my suggestion would be to give it a try. It's been two years and my hen is still alive and kicking and laying so I knew she wasn't an internal layer - which would be the other reason for the filling of fluid.

Could she be an internal layer or have some kind of intestinal or abdominal infection?

I too have 2-1/2 yr old hen with distended, soft, squishy abdomen below the vent. Thelma is different from these others in that she is laying either soft shelled or shell-less eggs and has been for months now....and she eats them when given the chance. She is happy, eats and drinks and interacts with the other chickens. I have clipped the feathers on her butt since she had poopy butt really bad, and I found this squishy distended abdomen with red streaks across the skin. The skin does not look thin but the red streaks remind me of stretch marks, only very red. I am cleaning her every few days with very diluted iodine and using a combo of Neosporin and Bag Balm on the reddened areas. Any suggestions? I started a liquid calcium for her today but don't have any idea of dose for a hen. Thanks for any suggestions.