My baby is teething at the moment and i've had to hold him and lay with him at night to comfort him to the point my upper arms get achy so i have to keep switching arms and sides. today my neck and arms feel v weak and sorta numb. could this be why?

Your pattern is not good for your health. A newborn can learn to self settle if you allow them to. It is the best way for both of you to stay healthy. If you want to do this until the baby is 12 yo continue. If not, let the kid learn to self soothe. Crying/fussing does not mean hurt. It is the way many react to the new outside world until they adjust to it.

Yes it could be. Teething is taxing for both parent and child. Your child has to learn to self-sooth. May I suggest for your baby's comfort and for your health and sanity that you take your child to see a Pedodontist, a children's dental specialist. No, it's not too soon. Your child needs support and comfort. You need sleep to have the strength to help. Call a Pedodontist.