Hello Erin,
It could be a number of things, and I really am not knowledgable enough to say, but I would start on some sort of anti-inflammatory drug now, and ice it for the first 24 hours, and then try some heat tomorrow. Don't keep it in the same position as it will have a tendency to become stiff. If you do have a muscle relaxant try taking it. If not give it over the weekend, and then perhaps be in touch with an Orthopaedist. I'm sorry you're hurting.

Feel better quickly,
Chris