Cause of period stopping after 2 hours.

[raquielle]

I'm currently an infertility patient waiting to go into an ivf cycle.  This will be my first ivf.  I was told to call my RE when my period starts.  Saturday morning it seemed to start.  Full red flow with cramps.  I called her and told her it started.  She called in my BCP pills and told me to start them on day 2.  2 hours after it started it stopped.  I've had nothing but very light spotting for 3 days now.  Of course I called her and she told me to hold off on the pills.  She asked me to go to the lab for a blood pregnancy test.  I'm still waiting on the results of that but am not optimistic since we have been trying for almost 3 years.  I've had weird bleeding before this as well and have turned out not pregnant.  What can cause this?