Question about getting pregnant and ovulation

Hi everyone.
We are a couple with a son who's almost 2.
We'd like a second child. I've been off the pill for 4 months now and have been trying without any other contraception for about 2 weeks.
My period started on 22/11 and lasted quite long! Until 30/11.
We had sex on Dec 1, 4 & 7.
Suddenly yesterday I got some bleeding.. first light blood, then dark.. it's not as much as usual but still.
Does anyone have an idea what this could be?
I also had some spotting last month, so..
Could I be 100% sure I'm not pregnant? Or is this my ovulation? (But that should have been yesterday, and isn't dark blood normal for that?)
Best!