What are the chances of a false positive?

So I'm due on tomorrow and took this test on a whim as af hadn't yet shown (2nd morning urine). Haven't had any symptoms apart from sore breasts, spots and an upset tummy. Stopped trying a couple of months ago, even went and got the pill for when I come on as I was getting so dejected... I just don't know whether to believe this, I mean are false positives common, or could this be real?! My head is all over the place, I dont know whether to celebrate or not cos I don't want to get my hopes up after a year and a half...