When you had sex on the 25, how long before that did your period start/end? You can have light bleeding/spotting in early pregnancy that some people mistake for a period so if i were you I would take a test just to rule that out. Are you having normal period symptoms, any cramping etc?
I've never heard of sex changing your cycle.. I'd take a test just to be sure... but sometimes your body just goes through lite times... you can also skip periods if you're under a lot of stress.
you can still have light bleeding when you're pregnant. please wait 2 weeks and take a hpt.