Periods problem. How can be this normal?

I had my medical abortion in September . October I have my periods. But November I missed my periods. In Dec 15 I had slite bleeding for 2 days . And on Dec 31 to till now am having my periods. It's not stopping . Meanwhile I have my pregnancy test 3 time but its negative all the time. So I consulted doc she gave me tab but still continues my periods. Aa week back I met my doctor and said till it's bleeding. So my said infertility and she gave me dupston tab for 15 days. Till now I have 7 tablets taken. For 4 days of using them my periods stopped for 4 days . Not completely but bleeding was stopped. But from yesterday onwards. My bleeding had started again. I don't understand what's the problem is . I asked my doctor she said it's OK. But am so worried about this my bleeding problem . It's about 20 days am having my periods. How can be this normal? I really need a suggestion please. Help me out.