I can’t help with the weight gain weight gain but I do know the pharmokinetics of reclast. Joint pain typically lasts 3 weeks or less but varies a lot from person to person. Have you considered that the pain is affecting your activity levels which is effecting your weight? Just a thought.

Within 24 hours of the infusion, the drug's concentration in the blood drops to less than 1% of its peak level. About 40% of the dose is excreted in the urine.
The terminal elimination phase has a half-life (the amount of time it takes for 1/2 of the drug to be eliminated) of 146 hours, so low concentrations of the drug remain in the plasma between day 2 and day 28.

The rest of the drug binds to bone, where it can stay for years. This is why reclast works for one or two yesrs without redosing. As I’m sure you know, bone is not static. It is constantly breaking down and being regrown. As the normal process of bone breakdown occurs, reclast is slowly put back into the bloodstream. I don’t know if these very low blood levels of reclast can cause symptoms. Perhaps someone here can help with that.