Gas pain, or something else?

So, my lower stomach hurts pretty badly if I tighten it. I'm not sure what to think it is. It's been persistent throughout today, and hasn't really changed. I'm concerned that it might be appendicitis, or etocpic pregnancy. I have no history of either, I'm 21 years old, and I've never been pregnant. No missed menstrual cycles or any other symptoms. I don't think I have a fever. What do those of you think who have had either of these problems? It's a sharp pain when I tighten the lowest part of my abdomen, and a very dull annoyance when I'm at rest. Thanks!