3 months 3 weeks and 1 day pregnant is 15 weeks pregnant. So 4 weeks later you were told you were 19 weeks. That sounds like you 'conceived 17 week prior to that - give or take a few days.

Women usually ovulate 14 days before their next period is due. That often works out to be about 2 weeks after the first day of their last period. This is not always the case, it is a general rule since women can ovulate whenever.

If this is a case of paternity, you'll need a DNA test.