My period came on march 8 2013 and lasted four day, itthen came on April the 18th and lasted three days and now its may and I haven't seen it on the 8th or the 18th could I be pregnant?

Sure you could but it could also be something else.

Are you sexually active? If so, yes, you could be pregnant. You'll have to wait until your period is at least 2 weeks late, then test using first morning urine. Sex = pregnancy, so if you're having sex, even with protection, pregnancy is possible. That's basic simple biology, and I'm really getting concerned that a large number of people don't seem to know this, or know that pregnancy can't be diagnosed online. It's mind boggling.

Assuming that you have engaged in sexual intercourse since April 18 then, of course, there is a possibility you could be pregnant. Wait at least another week and you can start taking pregnancy tests.