I had my last bleed come on the 28th of Jan (came exactly on day 28) and it lasted about 4-5 days, it usually goes for a full week (7days) and is super heavy, now 11 days after the first day of my bleed I've gotten such a sore lower back mainly on my left side. I also had unprotected sex yesterday (first day of my fertile week). Has anyone else experienced this? I'm 18.