What Causes Delay In Periods After Misscariage?

I took a pregnancy test that came out postive, and a couple days later bled out. I went to a planned parenthood clinic to see if i had miscarried, unfortunately the doctor said she could not see if the embryo was there anymore, so most likely i miscarried the baby, i bled out like a regular period and shortly two weeks after the miscarriage i started another period, after i let that pass i thought my cycle was going to be on track again however i haven't had my period for nearly three months, and i have taken several pregnancy test and it comes out negative so i don't know what wrong with me