Hooray for the negative test!! If your period is late, the test should be fairly accurate, but a blood test isn't a bad idea so you can completely rest your mind. At least when I was in grad school, the docs at the school clinic would give me a blood preg test any time I came in complaining of fatigue - so docs are very happy to do that sort of thing for women in their late teens and 20s.
Retrograde had some great links in her previous postings. It wouldn't hurt to brush up on some of those things. I just learned from a Planned Parenthood animation that fertilization is often *days* after you have sex - who knew? There are also loads of contraceptive options that most people don't know about, and also lots of great strategies how to renegotiate what activities you do or don't want to do with your BF.
--AC, with a 20-year spotless record of negative pregnancy tests