pain in top of hand

For 2 days now I have experienced pain along the top of my left hand, from base of fingers to my wrist. It's kind of a dull ache/stiffness with a stiffness deep in my wrist joint. This seemed to come on all of a sudden and is quite pronounced now. I did not injure it. At this point it's difficult to put weight on my hand and sudden movements are painful.

I'm only 25 so I don't want to think it's arthritis. I don't think it's carpal tunnel. Could it be tendonitis?

The only thing I can think of is that I get tension headaches and tense neck/shoulders associated with this. I had tension headaches all of last week, which have lifted now for the most part. Is it possible that this "traveled" to my hand?

I have a brace (for carpal tunnel). Should I use it? Is there anything else I can do?