Spotting

So I have pcos.. i have to take a tablet every 3 months to have a period. I took the tablet early this month and had my period finished it last week. The day after I finished I had unprotected sex with my boyfriend of 6 years. Yesterday I noticed I started spotting a little today it's getting worse/heavier. Getting some cramps. been taking ovulation tests but they still keep coming negative.. can anyone help as to why I could be spotting please..

Thanks