With a period starting on January 19, you'd likely ovulate between Jan 30 and Feb 4.
Sperm can survive up to about 5 days, the egg is only viable for about 48 hours after ovulation.
Your ex is very likely father. It was far too late on 17 Feb for your current boyfriend to be the father. You'd have already been pregnant.
Having sex on 17 Feb would make you about 16 weeks along - remember, it isn't immediate.
A DNA test can confirm.
Hope it helps.