Hi, I Am 18 Years Old And I've Always Had

Hi, I am 18 years old and I've always had a normal 36 day cycle. But I'm on my 2nd month of missing my period. I've taken plenty of pregnancy tests and they've all came out negative. October 8th was my last period and October 18th I took plan b, on October 26th I had some bleeding for 3 days, assuming it's from plan b. But since then I've had unprotected sex and I'm concerned since the home tests have been negative, that it may be something else?