If the last time my girlfriend and me had sex was February 17 and she is six weeks pregnant could it be mine after it being April 24?

Only a DNA test after the baby's birth will tell you for sure.

I can understand your wondering. Apparently you have reason to believe she may have been with someone else?

Pregnancies are first dated from the last menstrual period. It could be her last period was not an actual period however, and she just counted it as one and that is what they are going by for now. When she has an ultrasound, it will give a bit better idea of how far along the pregnancy is, and she may be a bit further along than first suspected.

However, as Wondergirl said, to know for certain, you may need a DNA test once baby is born.

Very possible since they don't date it from the sex but from last menstrual cycle.

There will be no way to tell, until child is born and there is a DNA test. Not info info on last period, to give a more detailed answer