Hi My Little Girl (12 ) Has Developed A Large

hi my little girl (12 ) has developed a large (nickel sized) water blister on her knee. she is a heart transplant recipient (12/2014) and currently has pappelidema (going for mri on tues) we don't know where it came from-wore loose pants all day, no accidents. I took her to er on tues for this and the dr was very nonchalant, popped it, and sent us home. tonight it is swollen again