How would I know if I’m being cheated on?

So my partner has been acting off lately, I’m 18 weeks pregnant so I thought maybe it was that, we’ve been arguing but things seem to get fixed, after our most recent argument he made a comment about sleeping with someone else, I hated that he said it and he apologised very quickly but it was still sad.

So basically I’ve been getting vaginal swabs every 2 weeks because I lost my last baby around the same gestation they want everything to be clear, my last swab was negative, now this current swab has shown up BV. Now I know you can get BV from other measures, but my dr did mention it can be sexual related. I keep my bits very clean and I’m very hygienic due to anxiety from my previous losses, so I asked my partner if he had potentially slept with anyone else, he went OFF and started yelling at me for not trusting him. Now today I found a red hair strand in his car, I have blonde hair. :/

I can’t talk to him about it because he just goes off at me for ‘not trusting him’ so I don’t know how I can get the truth.

Any help?