I'm sorry to say but this sounds like Marek's disease  and that lump will most likely be a tumour. I say this for the following reasons....
It is very common although most people don't get it diagnosed and/or don't want to admit that their flock has been infected  
She is a young pullet.... they are particularly prone to it.
She went mildly lame but was otherwise fine.... Marek's often causes lameness or paralysis of one leg, but they continue to look well and eat well.
It sometimes shows symptoms like lameness which get completely better or no worse and then they can suffer a second attack a few weeks/months later, usually when they are stressed.... maybe due to reaching point of lay or receiving unwanted attention from a rooster.
 
I currently have two in sick bay with it. The first is a pullet that went lame with it 3 months ago and got no worse so far. She happily hops around and looks the picture of health(bright eyes and red comb) apart from being petite and lame..... she holds it up like a flamingo with the foot clenched and facing backwards. The other went lame 2.5 months ago. Was lame for a week or so and at one stage was lying in classic Marek's splits posture. A few days later, she was back to normal and I thought she was fine but just as her sister has started laying, she has had a second attack and is now unable to stand for more than a couple of seconds and can't balance. She is bright eyed and eating well but not getting any better this time. I had another young pullet that went lame around the same time (2-3 months ago) and she deteriorated fast and I had to cull her as she also developed wry neck as well as paralysis of both legs. I found a tumour on one of her legs very similar to what you describe and another huge one the size of a duck egg on her abdomen when I did a post mortem. I have had 2 young cockerels of the same age get it but they have come sound again.
 
I am treating the recurrent pullet with turmeric and black pepper in the hope that it will inhibit the tumours. She is holding her own at the moment. I am thinking of making a frame and sling/hammock for her as she is lying down all the time and as a consequence, soiling herself. The hammock may also enable her to use her other leg a bit with support if I keep it low enough and/or enable me to reach under and do physio on her legs.   
 
There is no official treatment for Marek's and conventional advice is to cull as it is very infectious. Your other birds will now have been exposed to it so I wouldn't worry too much about isolating her now if she can be kept within the flock as this will also stress her. My sick bay/brooder is within the hen house, so that they can remain in contact with the flock but safe from being bullied.
 
What makes you think it is fluid filled and infected?
 
Personally I would advise against lancing the mass, as, if I am right and it is a tumour, it will not help and may lay the bird open to infection when her immune system is compromised as it will be with Mareks.
 
I hope some of what I have said will be of help. I am on a steep learning curve with this as my young flock just started showing symptoms late last summer, but if you have any questions about what I have written, please feel free to ask.
 
Best wishes
 
Barbara