negative then positive???

Hi so I took a pregnancy test this morning and tested negative. Then I wrapped up the test in its original packaging and put it in my car to throw away later. Hours later, I checked the test and it had a very faint positive line. I assumed it was just an evap line but I got curious and did some research. I found that some of these evap lines turned out positive for some people. I'm really scared, I can't be pregnant! I've been testing 2, 3, and 4 weeks after protected sex and all turned negative! Could it just be an evap line?