Try not to worry too much, first time sex can actually initiate your period due to hormone charges. It is quite normal to have an early period after sex, especially if you are not very sexually active normally.
Your hormones will soon balance out and your periods will return to normal. However, it is never a good idea to have unprotected sex, as you can get pregnant even if he withdraws before completion. Obviously there is always a risk of sexually transmitted infections too so a condom is always a good idea.
Just to be totally safe, it would be a good idea to take a pregnancy test. It's probably fine but it's always better to be sure. I don't know your circumstances or age but tests are reasonably priced and can be bought from any chemist.