I'm becoming more forgetful?

I used to be able to remember things easily, down to the tiniest detail. Since 8th grade, I've completely lost that. I always forget to do things even right after I was told to do something. Just this week I forgot to turn in two ignments and I got 0's on both of them, even though I did them. I forgot to buy my books for English cl, etc. I just forget the stupidest things. I take something downstairs and I remind myself to take it back upstairs, and I just leave it there until I'm looking for it. I've lost several expensive things (like my phone in Florida), and my mom's starting to get pissed at me. I don't know what's wrong with me and this is starting to get really frustrating.