Miscarriage. Worried how is it possible?

I missed my period on 3rd nov. So did hometest on 6th Nov. And it was faint positive. Visited dr. On same day and had beta Hcg test.. which showed the level 668. Thereafter on 15th. There was bleeding. In ultrasound, nothing could be detected. I again had beta HCG test. The result was shocking which stated a drop in HCG to 383. Thereafter dr. Said its early miscarriage and gave anti biotic. After a week, ultrasiund was dine and was given misoprost and other anti biotic. Meannwhile.. there was dark brown bleeding throughout this time. After these med. Nirmal bleeding started. On 30th I had heavy bleeding with clots. Thereafter, there was severe pain on 1st dec. Dr. Gave injection and ultrasound was done. The dr. Could not find anything serious bt said there could be chances of Ectopic pregnancy. Though now there is no pain. Bt as instructed by her, some nlood tests were done in which one was beta Hcg total oreganncy serum.. which was around 3900. I am worried how is it possible?