What Causes Vaginal Bleeding And Pelvic Pain Inspite Of Being On Birth Control Pills?

Question: Hello my girlfriend is taking the birth control pill and has not missed a day, taking it perfectly at the same time each day, shes also not taking any extra medication that could reduce the effectiveness of the pill. Also, she did not vomit or had diaharrea. We also did not have any problems for the past year or so (since when she started taking the pill). This cycle, we decided to have unprotected sex, and i ejaculed inside her since a long time ago (we have done that in the past, but i decided not to, to prevent) we had sex 4 days ago and there's left something like 10 pills to the end of the cycle, but she told me she had a breakthrough bleeding at the previous night and some unconfortable feeling/pain in the pelvic area (probably in the uterus?, nothing too harsh though) that started day that. The bleeding stopped a couple hours later, but today, during the day, she had again the bleeding, she said it looks like a period, but its not since there's still about 10 pills to go... She has been for more than a year on the pill and never got this side effect, i heard it is common in the first 3 months, but i don't know what it is when it happens an extended time later. Is this pain in the pelvic area connected to the problem (if theres a problem)?
I heard from another physician that she should take an estrogen pill for a couple months, does it mean the current pill lost its efficacy? or does still work? is she pregnant? whats happening given all the information i wrote ? .

One last thing, i was scared because i heard that some women bleed 3,4 days after the egg is fertalized, which is unlikely because she takes the pill, but still i got really scared, plus her pain...

Hello she didnt stopped the pill, shes still taking it, and the bleeding stopped now, her period will come about in a week, can she do the pregnancy test yet?

If is due to the ovulation, then shes pregnant? Or the pill doest allow it to happen?