A (good) family doctor will be able to tell you what is wrong by considering the following: What part of the forearm is affected? Is it the side near your little finger or the side near your thumb? Also, palmar surface or the dorsal (back of hand) surface of the forearm? Can you touch your thumb and little fingers together without any problems? He'll also likely do some sensory testing (sharp vs. blunt for example) to determine the degree of sensory dysfunction. Do you have any tingling in the affected region (paresthesia?)
I'm not a doctor, so don't take this as the final word, but based on the area you say you've lost sensation I don't think it's an ulnar nerve issue. The ulnar nerve doesn't really do much sensory innervation of the fore-arm, mostly the hand. With ulnar nerve entrapment the numbness would extend from the wrist down your little and half of the ring finger, and you'd most likely have some weakness in most of your fingers since most of the intrinsic muscles of the hand are innervated by the ulnar nerve.
With just the numbness you've described and no motor deficit I'm led to think that it's more likely your radial, musculocutaneous or medial nerve that are involved. You could have somehow impinged one of the cutaneous branches of any of those nerves or even have a deeper compression of one of them which so far is only manifesting as numbness.
Hopefully that helped a little. Now, go see a doctor!