First time posting - questions about cancelled cycle. Very confused!!!

Hi everyone,
This is my first time posting and I have a rather complicated question. I would SO appreciate some of your insights!
I am on Puregon (FSH) with a view to artificial insemination. I am 28 and not ovulating on my own due to a problem with my hypothalmus. This was my second cycle. My first was cancelled due to too many mature follicles. Same problem this time. I was put on the pill for two weeks. However, the day before I went on the pill I had unprotected sex. Last week I had a day of very light spotting. Today I went for a scan to see if my follicles had shrunk enough to start Puregon again. No go. There are still some big follicles there but my lining has thinned a bit.
SO, my questions: what was the bleeding last week? Was that SOME of the lining shedding? A friend told me she thought it might be implantation bleeding BUT if it was, surely my lining wouldn't have thinned and I wouldn't still have follicles, right?
Also, why aren't the follicles shrinking? My specialist didn't really answer me well when I asked. Is it possible I could be pregnant but he just didn't see it on the ultrasound because he wasn't looking for it???
Please, I am desperate for information!!!
Kate