There is a spontaneous bruising thing that is generally considered benign (but is a tad more common in something called Ehlers-Danlos Syndrome). However, it is not usually symmetrical.
Use your trusty iphone to document this swelling and bruising whenever it occurs. It makes a doctor's job a litle easier.
In terms of diagnosis, much depends on your age, sex, physical condition, and family history. You will almost certainly be OK until your appointment. In the meantime, write down as much as you can think of that might be relevant, and if you can, contact your family and ask if anyone else has similar symptoms. Heredity matters.
I would also like to add my back has been hurting for a couple weeks. When i breathe in it hurts on the left side near the shoulder blade. I sometimes feel tenderness on one of the rib cages as well.