This happened with me too... I didn't have any clots or anything. I just went to an A+E department and they sorted a scan to clear my mind. They too said it's very normal as long as there's no severe pain or heavy bleeding/clots
I had implantation bleeding with 2/3 pregnancies... I mean, given your history I think maybe they should just check your levels...just to make sure they're actually increasing as they should. BUT light spotting with no other symptoms can still be normal. Perhaps call around and get a few more opinions.