Have you tried taking any pain relievers, i.e. Motrin/Ibuprofen?
Sounds like a migraine in my opinion.
Any other symptoms?
Reminds me of an earache, altho I have to assume you'd know the difference. Also could be you got your wisdom teeth trying to come in, they can make the side of the head hurt, too. It's also possible you banged the heck out of it and forgot becuz it only hurt for a few seconds, and now you've got a little swelling and bruise. It's a remote possibility, but a tumor or burst blood vessel situation inside your skull can hurt, but usually you'll get symptoms that affect other parts of the body and not just pain in that one spot. I don't think you should be worried, but in an abundance of caution, go ahead and visit a family doctor or go down to the county health department "free clinic" as a walk-in and a doc will see you there for very little cost, or if the pain gets really out of control, going to the ER is also okay to do.