My girlfriend has been taking birth control since last october but for the last three months, she started taking it continuously.
The first month, as expected, she had an irregular period due to the hormone imbalance and the fact that she wasn't used to it.
The second month, she had no periods, so it worked.

However, last friday began her third period while continuously on the pill. We had sex without a condom thursday and yet friday, she had a very minimal amount of spotting! (she noticed red spotting early that day but said it was almost nothing and that it didn't continue during the day)
The spotting was not brown and she doesn't feel any different yet but I want to know if it is possible for a woman to have spotting even when on the pill continuously.

She first said that it was probably due to the fact that she took her pill a bit early thursday and a bit late friday so a bit of lining was able to release itself.
She also said that it might be due to irritation (she was irritated due to our very high sexual activity thursday), or it could be to accumulation of menstrual lining that has never been released.

Could any of those make sense?

Because if those things can't really happen, then I'm going to start worrying about implantation bleeding....although I didn't cum into her so....what are the odds right? :-(
Thank you for the help!