The thing about Internal Laying is that antibiotics will help them at first and even a couple of treatments they can continue to heal, but eventually since you can't stop the yolks, the E.coli infections get the best of the bird and no medication can save them. Other than having the ovaries removed, or an implant put in the bird, you can't stop the yolks from coming. You can slow them down by cutting way back on the protein levels and limiting their amount of daylight. Both of these things can slow or even stop the yolks. But it isn't a permanent solution to this situation. Internal laying is an awful issue that we can't really fix. I had 1 and only 1 bird ever survive internal laying. They say that some internal layers get this way from an infection in the oviduct that damages the walls and the yolks then fall into the cavities of the bird. Well I jumped on this bird after losing my first bird to internal laying and I got her on Amoxicillin fast. It took a couple of treatments, but her oviduct must have healed and she recovered. But I have had 2 other internal layers since then and I was not able to heal them, both had to be put down.
And your girl has obviously responded to the meds, but has had a relapse. Some birds are VERY sensitive to getting yeast infections on antibiotics, some not so much. I had a hen that after day 4, she had a ripping yeast infection and another bird with MS that I kept on incredibly strong antibiotics for 16 weeks and she NEVER ONCE had a yeast infection. So they are each different in how they work internally.
Many of these laying issues are E.coli infections and generally the infection is not contagious. The sick bird can be carrying some of it in their saliva or poop. I have never noticed my other birds becoming ill, however I was very cautious and used ACV or probiotics on and off in the water to help the others stay healthy and I did keep up the poop as much as I could. I did cage one bird that was internally laying that had a ripping doughy crop as I felt she was very very sick and I didn't want to take any chances. I put her down not too long after knowing I couldn't save her. Rarely to internal layers survive.
BUT...in the end, only a necropsy done by a vet or yourself at the time of their death do you know for SURE what they died from. Chickens can die from all kinds of things, some of them remain mysteries for ever.
I am sorry this may be your first loss.
My first passing was very hard on me. They are all my babies and gosh I felt I let her down. But chickens do contract things. It is normal to lose something like 5% of your flock a year?? Something like this. I lose a few here and there occasionally. I try and appreciate each and every day I have them, spend lots of time and tell them how much they are loved each day. We never know when their time is up, even in the best of conditions, they still pass on eventually.