Last weekend, my boyfriend and I (both 18) lost v to one another. We discussed a lot about it before actually doing it and we decided not to use a condom. He did not finish inside me, we took things slow and were both cautious about it. I felt slight pain when he entered at the beginning, and I know that's normal, but later on I felt no pain at all.

Everything was great, but after about 3-4 days from that I started having lower back pain and slight pain in my pelvic area, but my period is like 9 days away. My lower back hurts constantly and I thought it might be from sitting or laying down too much over the day, but I can't explain the pelvic pain. When I'm walking and moving my body I barely feel any pain or no pain at all, but while I'm sitting down the pain slowly builds up and mildly hurts from time to time, just like cramps while being on my period.

I'm afraid if it has to do with not using a condom, or maybe something else, but I am concerned. Does anyone have any idea why that's happening and is it normal?

Side note: I'm on the pill (for irregular periods) and have taken 2 doses of HPV vaccine. Both my boyfriend and I are 100% sure we have no STI's because we never had vaginal nor oral sex with other people.