Don't know if this has been resolved for you yet, but wanted to let you know I also have bloated and gained weight on the Flagyl. I think it's water weight because my weight has been varying +5lbs overnight. I don't know what to do for it, but wanted to let you know! I think any weight gain/loss that happens so fast is usually water weight, and probably won't be permanent. If yours came on more slowly and it sticks around, maybe get your thyroid tested as others suggested!
The bloating I have is just around my middle, not my face, arms, legs, etc. so it will hopefully go away once I'm off the Flagyl.