Screaming Nightmares?

I am 16, and I have been having screaming nightmares on and off for the last year and half (or about that) I'm not an unhappy person, I generally get enough sleep, I don't often take drugs and when I do it's the odd puff of a joint, very rarely. I don't drink, And nothing in my life is making me worried or stressed. I have exams soon but I'm not worried about them as I do well in school, and If it was the exams causing the nightmares that wouldnt' explain the last year or so. I don't remember the nightmares, Just a flash of a picture, a huge spider hovering above my head. I don't like spiders, but I'm not scared of them so it makes no sense for me to be so scared of them in my nightmares! I really need to work this out, as It's starting to piss iff my mum and her fiance that I wake them all the time :/ Sorry that was so long! xxx