Please help, cud she be prego?

My gf had her baby feb 2011, last apr4-8 she had her first mense we had contact on apr 18(fertile) but we used withdrawal (were not ttc), she had mense again may 4 and 5 (only lasted 36 hours) she said it is like a period and she changed her pads 6-8times with almost full pads (could it be implantation bleeding?) we had hpt on may 2, may 10, twice may 14, may 15, first morning urine may 23 and may 27 all negative, rushed her may 15 at hospital complaining of severe pelvic pain, (a friend say it might be ectopic) doctors done urine test negative, also done 7 dilutional (not familiar with this one) also negative, transvaginal ultrasound done they say she is not prego, they diagnose of a possible PID, prescribed with doxycycline 100mg and metronidazole 500mg for 2 weeks, then we head home and both got shots of ceftriaxone IM, we were for follow up on may 23 had lab test for STD (RPR, HBcAG, ICC Elisa) all negative, she is not having any symptoms and she had her mense this month June 2-5 and we had another pt today june 7 ,she said it was negative, cud she still be pregnant were not ttc thank you