pregnant or psychosomatic symptoms?

well..
I'm pretty scared I'm pregnant.
I'd like some insight, though, because part of me wonders if I'm over-reacting and looking for symptoms.
I DID have unprotected sex, so that's a start, I suppose. stupid kids.
anyway.
the last time I got my period was September 15th. it was on time, like always. I'm so regular, it's probably ridiculous. but it was different than my periods usually are. mine always last exactly seven days, and last time it was only five and a half. the cramps I got felt much different than they usually do, also, although I can't really describe what they felt like. they just didn't feel.. normal.
I've been nauseous, but it isn't like what's normally described as "morning sickness." it doesn't come at a specific time of day.. it just that things make me nauseous much easier than they used to. I've never ever been squeamish or easily grossed out, and lately even little things make me just gag.
I haven't been having any weird cravings, but I have really desired to eat some things I really don't like.
I've felt constipated and bloated for about a week, and I feel like I look chubbier (but I can't tell if I'm just imagining it.)
my hair has been getting greasy really quickly lately, which isn't normal, and I've been breaking out a bit, which isn't normal either. and my nails have been growing freakishly fast. I've heard those can be early signs of pregnancy, too.
and just tonight, I started feeling a strange, almost stretching feeling under my rib cage. it's not extreme or painful or anything, but i wouldn't say that it's subtle either. I don't know that if I were pregnant, I would be far along enough to feel that sort of thing.
my nipples feel the tiniest bit tender, but I'm not sure if that's a psychosomatic sort of thing, because that happens to me in general sometimes.
I've gone to the drugstore a billion times this week and stared at the pregnancy tests, trying to decide if it's worth buying one now, or at least waiting to see if I get my next period and what it's like.
I'm really terrified even just to buy it..
the few close friends I've told think I'm probably over-reacting, so I guess I'd just like some suggestions.
thanks a bunch.