What Causes Brownish Vaginal Spotting Along With PCOD?

i have been diagnosed with PCOD. Started on Meftformin 1st week april. Did s TVS in Day 12 (April 10th), took HCG inj 5000iu IM got my periods on May 4th.. and it stopped after 3 days. 2 days after that i started bleeding again and since then have spotting sometimes fresh blood sometimes brown discharge. Consulted my OBGY on 11th May,TVS repeated ?small sac seen; was started on Duphastone 10 mg tds. but im still bleeding.. why?? urine hcg was very weakly positive