Could I be pregnant? I had sex, then got my period a full week early.?

Ok, so I've always been very regular to within a day or two with my periods. But I had sex, then had a period a full week early. Then right after my period ended, we had sex again, and now only 4 days later I find I have very light bleeding, no cramping like with a period though. It's only been 12 days since my last period started. After the first period was early I took a test, but it expired in 2008, but it was negative. Should I take another one? I don't have nausea or anything, so... what gives?