Most women ovulate about 14 days into their cycle, so if he didn't pull out on the 12th or 13th, there is a chance you could've gotten pregnant. The spotting could be different things, and sometimes women spot for seemingly no reason at all.... It could also be implantation bleeding. Sorry if that's not what you want to hear!
Try not to worry about it though. If your period this month is even one day late, I would take a home pregnancy test (because if it were me, I wouldn't be able to wait any longer than that).
Since you've already had a child though, if you are indeed pregnant, you might begin having symptoms before your period is even due.
I hope all works out for you!