I would say u should just get that DNA testing if if u wanted to b for sure.
Yu have to go back about 41wks cuz yu don't get pregnant right after sex yu have to give it time the best thing is a dna test maybe yu can have it done secretly if yu don't want any of the guys to know
One of my friends had their baby March 26th and conceived in June so it's your current bf.
As for the green birthmark both my sons have them. Their called Mongolian Spots.
"Mongolian blue spots are common among darker skinned persons, such as those who are of Asian, East Indian, and African descent."
hope that helped
You don't need to test if you don't want too! Seems your current bf is the father, I'd just leave it at that!
You can count dates as much as you like...but there's no way to be 100% certain without a DNA test