Help? :(

I'm a new memeber and i'm not sure what type of curl I have (they all seem to fit in one way or another), but I'm having a serious issue with grease or product build-up (I'm not sure). :sad5: What happens is that I take a shower- shampoo my scalp, then condition the tips and brush the tangles out- and afterward, when it starts to dry, I notice that the center of the back will be kind of sticky to the touch (the place where older men most commonly get bald spots is the only way i know for sure how to describe where this happens). I'll rinse it again, but it doesnt seem to make a difference. It really bothers me, because it's not only affecting the volume of my curls, but it seems to creep down my scalp a bit and make my hair matted in the back. It also sometimes looks like it isnt curly at the top anymore sometimes because the "stuff" is weighing it down. :sad5: My hair has been curly for about 5 years now (it changed around the time I hit puberty) and I've never had it do this to me. My only problems in the past have been frizz. I don't use any other products in my hair besides shampoo and conditioner, and that has always worked wnderfully for me......any ideas what I can do to get rid of this mystery "stuff" and get back my otherwise delicious volume?

<3 L.L.