Period after sex?

Hello! I haven’t had my period in over 4 months (always have been irregular, dr won’t prescribe BC due to having RA and getting it situated w a diff doc) me and my SO had sex Monday evening. I started to bleed dark brown for a few hours, and I thought we were just too rough. It’s Wednesday evening and now I’m actually bleeding, and it’s bright red. I’m cramping and having back pain, but could sex have triggered my period after all this time? It’s curious! Thanks ladies! XX