I can move it, but with varying degrees of pain when I do so. The faster I move it, the worse the pain. It's a sharp pain. When i'm not moving it, there is no pain.
Inagine if you will, reaching out to shake hands with someone. Your arm is extended and you hand is open and ready to receive the hand of the other person. From this position, turn you hand slowly so that it is now facing the ground. That is very painful to do and the pain is originating from the muscle on top of the forearm between the wrist and the elbow. But funny thing is I can squeeze my hand tightly with no pain at all.
That help any?
You likely have a partial torn ligament or ligaments. The ones which allow you to rotate your wrist and raise your hand. But the finger ligaments are likely OK. This is a common injury.
So long as you can make all your normal movements, the injury is likely just a strain, not a separation of the attachments to the bone and muscle fibres. While this can self heal it will take some time.
You have mentioned nothing about bruising or skin discoloration, so I presume internal bleeding is not an issue.
It will also be annoyingly painful and inconvenient. Again depending on severity or pain, limitation of movement, and current symptomology presented, I would suggest an emergency visit is not required. But that is your call.
Alternate heat and cold compresses on top of the muscle for a few days. Take aspirin to reduce inflammation. If severe symptoms persist for more than a few days then I suggest seeing a physician.
Again, I'm not a Doctor. But, based on what you have described, this is how I would handle it.
It will take about six weeks to heal so be tender with use of the arm. But don't immobolize it entirely. Again,for a few days, put a tennis ball in your hand and wrap it and the arm with an ACE.It will hold therotating tendon(s) in a relaxed position.