Neo I experienced implantation bleeding with this pregnancy. It was brownish red & only lasted 1 day, was not heavy at all. This was 11 days after conception. I didn't have any pain or any period symptoms whatsoever with it. So this is different to what you're saying happened to your girlfriend.
Maybe what your girlfriend experienced was a period, possibly her cycle is out of whack from the pill changes? But I am not an expert & in my opinion she should do a pregnancy test to make sure, because if it was an implantation bleed she would be able to get a positive result by now.
Hope this helps!