irregular periods

I was on depo for years and then off for a year then got pregnant, had my daughter in May. Had one depo shot afterwards and bled for 4 months straight. So I obviously didn't get the shot again, because who wants to bleed that long?! Well now my periods are coming more frequently, sometimes two weeks apart and now I had sex on the 3rd and started bleeding again the 10th, which I wasn't due for a period for another two weeks, I had sex in my fertile window. My question is how is this normal? Which my doctor tells me. And why so much bleeding?