Is this normal?

I’ve recently noticed that every time my boyfriend and I have sex, I get my period a day or two later. At first, I thought it was just coincidence but there has been a pattern with days that we have sex and the start date of my period. Is this normal or is there something wrong?

It may be worth noting:

I’ve never had a regular period. In fact, I started my menstrual cycle when I was around 9 or 10, and then I didn’t get my period in almost two years.

I started taking birth control when I was 14 and it came back. After about a year of taking birth control, I was able to bleed on my own again. My cycle is still irregular. Sometimes it lasts about 8 days. Other times it can last almost two or three weeks. The timespan in between periods is just as random.