How Soon Can I Take A Pregnancy Test Post Unprotected Sex?

21, Female - My husband and I are TTC and my last period was December 31, 2009. I took my first HPT Feb 6th and it came out negative so I tried again in the morning hoping for the higher hormone level and got another negative. I decided to wait a little longer to give it time seeing as how I have had a little stress the past month. Around the 8th I started to spot lightly (brown and only notice when I wipe) and a day later it was gone. I re-tested the 12th and this morning and still a couple of negatives. I have experiance lower back pain and some unusual feelings in my lower abdomen along with fatigue and some headaches the past 2-3 weeks though it could be the lowering of my caffeen in take. The last couple weeks have been an emotional roller coster ride and I just want to know if I should go and see a doctor asap, or if I should just continue to wait it out.