Pregnancy test results!

So I built up the courage to go and buy a £1 test . I see the results as negative ( what do you see) is it negative or invalid. I think I'm overdue my period but can't be 100% sure. In spotting atm but I've never had this before my period only after occasionally. What should I do next ? Do I test again ? Go to the GP, I always have super light periods that last only three days . My gut has told me the whole time I wasnt pregnant but I don't know what to do next .