Trying again...I donated blood a month ago and my arm/inner elbow is still weak and it's painful to pick up heavy objects. Not a derm problem.?

See your Dr. Start w/your dr for good exam of elbow and possibly imaging or initial testing to guide follow up--while complications from blood donation (or iv) in a/c space are very rare, can happen ; need neurology or orthopedic follow up. If no primary dr, can start w/neuro as is more likely an irritated nerve (that often runs near blood vessels) as source.
Doctor to evaluate. If the area of pain is exactly where the blood donation needle went through, then it is likely that the pain is related to the needle. A doctor can examine the area and see what might be the problem. He will want to know if the needle insertion was an easy painless insertion, or if the technician needed to move the needle around to find the vein. Hopefully, there will be daily improvement.