Could my girlfriend be pregnant?

Me and my girlfriend are both 16. We haven't had vaginal sex but we have had anal sex. We used a condom, but it broke. We made sure that no semen got into her vagina, but about half an hour later, she said she felt fluid in her underwear. The fluid was thick and transparent, and we thought it may have been vaginal discharge, but then we started thinking it may be semen leaking from her bum. She got in the shower and tried to wash her vagina as best as possible just in case. Its been a few days now, and she has been getting dark brownish black vaginal discharge, also, her breasts may have grown and she says her appetite has too. Its not long till she is due to start her period, but we are getting slightly worried. She has IBS (irritable bowel syndrome) and her periods are very unpredictable, they're usually two or three days early or two or three days late, but we are wondering if anyone could help us by answering if they know if it is possible for semen to survive in an anus for half an hour? and if so is it possible that the semen could then swim up her vagina from her underwear and cause pregnancy? and finally, what does black/brown discharge mean?