I would trust the first, positive one. If you're not late for your period yet then there's a really good chance that you just didn't have enough hormone in your urine to get a positive on the second test. You should test again tomorrow morning with your first morning urine, that way it'll be concentrated - if the test is negative again then it's possible the spotting is due to a chemical pregnancy (a very early miscarriage). If the test is positive, then the spotting may have been implantation bleeding.
Either way, if the bleeding gets heavy and the pain worsens, call your doctor or OB and let them know (and make sure they know about that first positive test).