HELP!! My horse has a huge (half gallon) sized liquid or puss under her skin on one of her hind legs?

In the ox region. I noticed three days ago that area seemed to jiggle a bit more than the other when she galloped. But I just came from the pasture and it has grown tremendously in size (still in disbelief!) I put her in a stall. I just dewormed her and gave her shots (flu, tetnus, etc) about two weeks ago. Is this lameness? Isnt it usually in the lower leg area?? Please help, this is really worrying me! She is a 15 yr old tennessee walking horse mare.