Why does my chest hurt?

I am 28, female, and not overweight. No close fam history of diabetes or heart disease... The chest pain is sharp and stabbing, and I really don't believe it to be heartburn because it's so localized to my chest. It's sometimes more burning or sore feeling, but usually more sharp. Sometimes it'll go into my shoulder joints briefly. It has been going on for 2 weeks some days worse than others. Often it's worst at night and hard to get comfortable. It has been mostly on the left side of my chest, just to the left of center. I haven't had any trouble breathing or cough associated with it. Any ideas?