You could be in the clear, but...
Some women still have a period in the early months of pregnancy, so you'll only know for absolute certain by taking a test. If you're having unprotected sex, assume you're trying to get pregnant on purpose. If you want to avoid pregnancy, use at least two forms of BC (like a condom and a spermicide, etc.). There's no point in having unprotected sex, then getting worried later. Just avoid that unnecessary fear. Protect yourself in the first place, and skip feeling this way. Don't put yourself through this.
Here's info on when and how pregnancy tests work:
http://womenshealth.gov/publications/our-publications/fact-sheet/pregnancy-tests.cfm#a
