15 day period

I've been on BC for about 5-6 months now- progestin only pill. My periods have always been regular. I missed one pill and had unprotected sex two days after so I took the EC pill. My next period came when it was supposed to but it was much lighter than normal. My nurse said it was because of the EC and she did a pregnancy test which was negative. After that period I got another period 12 days after. It has been going for 15 days now. Anyone know what this could be or has had something similar?