If you have talked about a lost tampon, and if a doctor knowing this has done a pelvic exam with a speculum and a light, he or she would have seen the tampon. Obviously something is going on, but it seems unlikely that it's a lost tampon. (An ultrasound will probably not help even if the problem is a tampon.) If you are having repeated diagnoses of BV, talk to your doctor about your partner. Has he been treated with the same antibiotics as you, at the same time?
Ugh. Hard to say. I had a forgotten tampon I'm embarrassed to admit. Really, I now recognize these things can happen to anyone. I wasn't drunk, just a tired mom. lol I had some funky, thin watery but black discharge that smelled really bad. And the thing is, gravity can help you figure out if it is there. Reach up while baring down. Since you are seeing an ob/gyn, tell them your suspicion. The thing that makes me doubt it is that you feel that it would have been a years time. I think you'd be in worse shape than you are if a tampon was there for a year. Anyway, come back and tell us!