Does medication for bipolar or any anxiety cause weight gain ?

Like a lot of medications, some for bipolar do and some don't. Same with anxiety meds. You're assuming you are bipolar, and you're worrying about medications for it, but you need a psychiatrist to give you a diagnosis. I think your anxiety is causing you to worry about a lot of unnecessary things. You realize that worrying about gaining a pound is obsessive and not normal, right? I think you'd benefit from talking to a psychologist about your anxiety and your huge fear of gaining weight.