It's possible to get a false negative, but not a false positive, so it sounds like you are pregnant. However, are you getting a faint positive within 3 minutes of doing the test - or is it taking a long time to come up? If you check the test after 10 minutes then a positive result can be an evaporation line, which isn't actually a positive but just the used test ageing in the air. You should only read the results within 3-5 minutes. If it's saying positive then, even very faintly, then it should be correct!
If you're not sure then try a digital test like Clear Blue Digital, instead of a line you get a screen which says "pregnant" or "not pregnant" which is much clearer to be sure about! I had to do this, as my first test came up very faint and I was confused. I didn't dare let myself think I was definately pregnant until I got the Clear Blue test and saw it for real - very tense 24 hours!