Yeast infection 😩

I keep getting yeast infections, I don’t know why, I wear cotton underwear, I don’t use soap down there, I do everything right with keeping myself clean. My doctor thinks it’s possible that I have an autoimmune disease. I am lost it’s suck I’ve 4 in the past 3 months, or could it be my significant other? I’m lost. Please be nice my anxiety is horrible.