Do you tihnk my girlfriend is pregnant?

My girlfriend had her last period on April 5th (last month) and hasnt gotten it since. Normally I would be loosing my mind over such an event, but we haven't had sex at all since then. The only thing that we've done is orally pleasured each other. There has been no penetration what-so-ever at all. She has probablby the biggest test of her life tomorrow, and assures me that everything is fine. Could she have missed her period due to stress? Is it at all possible for her to be pregnant? So far she isn't showing any of the typical signs other than the fact that she missed her period. She tells me not to worry, but isn't a missed period something to freak about?