Maybe you just overdid on Saturday. A common thing to do when you are feeling well after being sick so long. Your body has been so stressed by illness that it just isn't going to bounce back to full steam ahead. Try to pace yourself. It's going take a few trials and failures to figure out just what you can do and for how long as you continue to improve. So don't feel like the treatment failed yet.
And, while it is nice to have a goal to fit into a bridesmaid dress, if it doesn't happen, it doesn't happen. Your health is more important. Also they may be able to alter it or you can get another dress in the same color if your friend really wants you in the wedding party.
And for your mental health's sake, give yourself credit for all that you are doing and have done. Don't focus on what you haven't done. Just causes anxiety, depression and hopelessness. You've really done a lot researching and getting tests and treatments, working, taking care of the wee ones, etc. Your weight is not your fault either. So many of the drugs you have taken cause huge weight gains. I'd be a blimp if I took them.