I think it's stress related. Do you feel it in your back?
So weird, I'm experiencing exactly what you're describing, al462! I'm 21 years old, and a college student, I am wondering if it is stress related? Or if, perhaps, I've pulled something on that side. It makes no sense. It's very dull and subtle, but still there. I think the more I think about it, the worse it makes everything. I've always been somewhat of a hypochondriac. :-/
It's been only 4 days and you don't have any real pain, only slight discomfort.
Unless symptoms worsen, I would wait to visit a doctor for this.
It might be internal bruising or inflammation that will heal by itself.
If symptoms persist or worsen you should definately get it checked out by a doctor,
or perhaps even a cardiologist.
The symptoms your having right now don't seem like anything to be worried about though. ;)