I Missed My Period Im Having Lower Abdominal Pain, What R The Possibilities ?

i had my last cycles on 7th june, had sex on 22june, on 7 july took the hcg test that was negative, had a chest xray on 8 july, repeated hcg test on 10 july that was a weak positive, had an ultrasound showing thickening of the endometrium but no gestational sac, doc gave me primolut n 2 tabs a day for 5 days on 15july, along with that took 375mg augmentin 5 doses due to sore throat, now its 24 july, test of hcg was positive 2 days back, have nausea and no menses, bt from the day i missed my period im having lower abdominal pain as it occurs bfore menses start and im still having it, what r the possibilities ?