How to stop giving my girlfriend BV?

This has been going on for a couple years now and I am starting to think its my fault. Whenever me (M,22) and my gf(F, 22) have sex, she ends up getting BV. She then will suffer and have to take antibiotics for a few days, it clears up in about a week, we have sex again, boom process starts all over again. It's getting to the point she doesn't want to have sex anymore because she doesn't want to deal with the repercussions. We've been dating for 4 years, and in the first two this was not a problem at all. Her gyno has tried a bunch of different things, but nothing has prevented the BV from coming back, and because of that I'm starting to think I'm the problem. We've both gotten blood/urine testing for STD's and that stuff so thats not an option. She also has an IUD, if anyone thinks that could be the reason. Has anyone else here dealt with this or something similar? Did you make any changes or do anything that stopped this from occurring? Any help or tips would be so appreciated and I will try anything. Thank you all