Hello~Has any doctor taken an x-ray of the arm to see if possibly you have chipped or cracked a bone? These could cause the pain you are having.
As to hitting a nerve drawing a blood, it could cause it, but it would have been going on the entire time, not coming and going.
One other thought, possibly when you hit that bone, you could have misplaced a vertebrae in that area, seeing a chiropractor might help, he/she would take an x-ray of that area as well as the upper spine and neck, as they could also cause the issues you are having. The chiropractor would study the x-rays and your history, then go over the results with you. A few treatments should have the pain cleared up.
The American College of Radiology (ACR) offers the following evidence-based recommendations: Initial evaluation of chronic elbow pain should begin with radiography. If radiographs nondiagnostic but suspect nerve abnormality, consider MRI elbow without contrast, or alternatively US elbow. The ulnar nerve is particularly vulnerable to trauma from a direct blow in the region of the superficially located restricted space of the cubital tunnel. Anatomic variations of the cubital tunnel retinaculum can contribute to ulnar neuropathy. Some sequences on MRI can depict the size and shape of the nerve, while other sequences on MRI may show abnormal signal in the presence of neuritis. Both are more sensitive than the conventional nerve conduction studies which were performed by your neurologist. US may also show ulnar nerve enlargement and increased vascularity and, when added to electrodiagnostic tests, increases sensitivity for the diagnosis of ulnar neuropathy at the elbow from 78% to 98%. A snapping of the medial head of the triceps can cause recurrent dislocation of the ulnar nerve. This diagnosis can be confirmed with dynamic MRI with the elbow in flexion and extension. US is ideal for dynamic assessment of ulnar nerve subluxation and dislocation, as well as for confirmation of snapping triceps syndrome. Radial nerve, median nerve, and other entrapment syndromes can also be evaluated with MRI.