After your first time having sex is it usual that your period is messed up?

I would say no. But what do you mean by messed up? Is it lighter, heavier, gone altogether? Women, especially girls still developing (and they do up until they are about 18), go through fluctuations in their cycle which is normal. But if its unusually light or late, consider buying a take home pregnancy test, or buy two to make sure you had an accurate reading. If your late on your period wait it out a few more days, stress can cause a period to delay. And I don't know about you, but when I first started having sex, I was really stressed and worried about getting pregnant, but usually, if you use a condom the odds are on your side. So try to relax and see what happens.