I wasn't IVF, but I had very light, barely there bleeding until 6 weeks gestation with DD in 2010, and then it occured every 2 weeks after that until I hit the 2nd trimester.

Out of curiousity, when was the last time you has an internal u/s to measure your endometrial lining? Is it 'too' thin, and shedding away? That happened to me after being on the OCP for 3 years (before kids).