Have you checked the side effects of the meds you are on? I have taken some that have blown me up like a balloon, and after stopping those meds, continuing with my exersises (a little more often or a few more reps) the weight has dropped off again. They checked my Thryoid first thing and there was no problem there. I did change my eating habits when I could not move around as much as I used to. I know you hear it over and over again, but a healthy diet, free of preservatives, etc. is what you want to do. I am a veggie lover so there is not problem for me having meatless meals. Just required a bit of getting used to. Best of luck!
Are you certain that you have not changed your diet or exercise regimen? Water retention can temporarily cause weight gain, but this is most likely from too much salt or changes in hormones.
If it is a thyroid issue you would most likely be noticing fatigue, sore joints/muscles, swollen neck area, bowel issues and/or changes to your skin and hair.
Sometimes it is the little changes in diet that gradually affect our weight overtime, so take a good look at what could possibly be the culprit.
If you are still at a loss, it can't hurt to get some tests done.
All the best.
Please consult with doc to get your thyroid checked.