Why My Period Is Delayed?

On May 28, 2011 my boyfriend and I spent time together intimately, though we did not participate in intercourse (we are both virgins). The tip of his penis came in direct contact with my vagina, though there was no penetration nor ejaculation. The first day of my last period was May 8th, and although I've never been completely regular with my cycle, I'm always able to at least guess within a couple days as to when it will show up. I'm about 1 day late currently, and I'm worried that I could be pregnant? I've taken one at home pregnancy test and it came up negative.