Hi.. The cause for late peroids could be due to the minor alterations in your lifestyle recently or secondary to an infection (as you are with a physical relation ship with one).. Most of the time they resolve after a few days.. If you are very apprenhensive of pregnancy (which is very unlikely), you could get a home pregnancy test done.. That would be the best and cheap way to get over the apprehension.. But if these the problems persists, you would benefit by a physical consultation from your doctor...