With my first, I got a faint positive with a First Response and went to the doc to have it confirmed. I had a lot of water to drink, so my urine was diluted and the first test I took there was a negative. Then I waited about 30 minutes and tried again and got another faint +.
With my second, I took a First response and got a faint +. The next day I took the other one from the same box and got a -. So I bought a couple different kinds (I had saved my FMU in a cup) and tested. i got another negative with answer early but a positive with a digital test (I think clearblue, but I'm not 100% sure). I had a blood test within the hour and had an HCG of 46.
When you're really early in your pregnancy, you have a pretty low amount of HCG in your system, and even though the tests may say they test a certain level, they test anywhere up to that level. So possibly with your first test your urine was more concentrated, or the test registered at 10 mIU's or something. Or maybe the second test was faulty. You're much more likely to be pregnant with even a faint + than you are to not be pregnant. Good luck, and congrats!