I have increasing muscle pain and weakness in both arms and back of neck for about a month now. What can cause this.?

Lots of choices. Stress, overuse, spasm, electrolyte imbalance, etc. Needs a good history and exam by your dr or urgent care to narrow down and make good treatment plan. For short term, can try local heat, but don't postpone dr visit.
Lots of choices. Stress, overuse, spasm, electrolyte imbalance, etc. Needs a good history and exam by your dr or urgent care to narrow down and make good treatment plan. For short term, can try local heat, but don't postpone dr visit.