Intercourse at the end of haidh

I am a married lady. I need to ask about sex in menstruation. I had normal seven day period in my life but since my child birth (3 months ago) I have had 2 periods; 2nd period was 8 days before due date on the completion of 7 days of menstruation I thought my periods are finished but I didnt take ghusl yet for purifying myself and I had slight spotting again on the 8th day morning. Thus furthur delayed ghusl and then had sex in the evening thinking that periods would definately be finished by now and I will take a bath after it. Then next day morning I had spotting again, only minor one which was shocking since I didnt take ghusl and my menstrual spotting lasts more than usaual 7 days this time. Have I really committed a haraam act? If yes then whats the expiation?