How common are false positives?

I'm not sure I want to be pregnant and it's our own silly fault for not wearing contraception urgh. I've had a very faint positive on a test, I'm not making it up as I've been here before and edited photos to see if there's the tiniest line. It's there. But it is faint. I'm due on my period in a few days, and from what I remember the line was darker with my dd at this point. Could it be a false positive or a chemical? I'm not wishing it away and I know lots of women are dying to be pregnant but this makes me really anxious, I have a lot of ptsd from previous pregnancies that failed.