why do the thoughts and feelings feel so real before my period starts

Yes. I feel that deeply and at the end of it.

Literally going through this now. My urges to look up triggering images/content is super exacerbated and the images in my head are constant.

it's normal as a female for us to go through different emotional periods with our cycle. sounds like your OCD is strong during this time because of your hormones. i would suggest keeping a journal near and keep track of your cycle and when your OCD thoughts peak. that way when you start to get very obsessive again, you can say to yourself - oh right, it's because of where i am with my cycle. that way you can be prepared for a spike and take control instead of giving in to your ocd!