What Causes My Periods Get Late?

Hi. I was taking IVF medication as a potential egg donor. I took leuprolide for 16 days, and I took FSH for 6 days. My follicles weren't good enough so they canceled it. My last period was may 10 (the day my fsh started). The exact day it was cancelled, I had light spotting around 3pm (but the blood seemed brown) only for an hour. I hadn't taken the shot that day, but I normally took the FSH shot at 4pm. Also, I have just 4mm uterine lining. Now it is June 10th and I still haven't had my period. It's only about 3 days late, but normally I'm like clockwork. I took a pregnancy test today and it said negative. What's the reason for my late period?