Arm weakness aftering carrying objects?

Sometimes if I'm carrying something, once I put it down my arms go really heavy, can barely move them (takes a lot of effort to even hold my phone or a bottle), they will also shake very noticeably for up to an hour after. It doesn't have to be a very heavy object. Two examples are a 30litre bag of cat litter but another time it was my purse, phone and a water bottle (so two very different weights). My doctor told me to do exercise to build up my arm and wrist muscles but I… read more