What Could Short, Heavy Periods Post Unprotected Sex Followed By Cramps And Nausea Indicate?

Hi, I m worried about my girlfriend. She was on her period about 2 weeks ago and it ended before July started. After it had ended, we had unprotected sex (didn t ejaculate inside). But to be safe, we had gotten a Plan B pill and she took it. About 2-3 days later, she said her period had come back heavily (red/brown colored), not spotted in pink. It had stopped after 2-3 more days. And now (1.5 weeks later) she feels somewhat nauseous, slightly fatigued, with stomach pains/cramps and light headedness. No frequent headaches, just once in awhile. Only back aches from moving boxes. Her breasts have not become swollen/tender, nor have the nipples gotten any darker She DOES take medication that affects her mood slightly (she s epileptic). And she is under some stress and depression. Is it possible that she is pregnant? Or could it be a mild case of an infection?