Hey everyone.....

This may be a stupid question but how early (DPO) can you get a BFP? We arent really trying atm but I've been feeling a bit bleh during the night (its waking me up) for the past two nights. So this morning when I got up i thought what the heck, and POAS. But, I didnt watch it because I was obviously expecting a BFN, so had my shower instead. I got out and got the shock of my life, a really faint line, like its barely there, a shadow that was slightly darker on the pee end. Question is, how quickly can evaporation lines appear? I wasnt in the shower THAT long, but did wash hair etc so maybe 15 mins? I used a spare Crystal Clear one I had laying around, the system one, not a strip, and i checked before, its still in date. Thing is I was only 6 DPO...... is it even possible?
Any advice would be great, I've been mulling over this one all day.....I really dont want do go out and buy more tests yet because I will seriously go through heaps until I get an answer one way or another......