brachialis and shoulder pain in left arm

[AdamGB]

Could be an impinged nerve in your shoulder girdle with radiating pain.

The majority of boxers have a massive imbalance in the shoulders... Overactive and overdeveloped anterior deltoid/pec minor and a weak and inactive posterior shoulder muscles.

Start doing deep pec minor and anterior delt stretches/soft tissue release and strength exerses that involve retraction of the shoulders and/or external rotation, such as bent over row and reverse fly. This will open up the subacromonal space in your shoulder and should free any impinged nerves or tendons.

A wall bicep stretch may help too

[Gray Lion]

I am not a doctor. There are a lot of questions unanswered....age? past injuries? Your training techniques? It could be anything from minor to something a doctor needs to check. It could very well be that you need to lay off training for awhile to heal a tear, bruise, or injury. If it really hurts, you need to have it checked out. Only You can know.