Low period flow.

I used have a normal period cycle every 5 days a month but recently (before I became sexually active) my flow has decreased to a little or to none out of the 3 days. I’ve gone to the doctor and asked about it but she didn’t tell me anything about it. Fast forward today when I’m sexually active, my period is back to normal. The first 2 days are heavy, the 3rd- 5th medium or low. But my boyfriend and I were being adventurous and we decided to have sex while I’m still on my period (it only happened once). After we did that, my period decreased again. I don’t know if it was the sex that cause my period to have a low flow or something but I don’t know why my cycle changed drastically. I know it’s best to go to the doctor and ask about it but I would like to get a second opinion or maybe advise about it. Thank you.