Left shoulder dull ache sort of feels like my shoulder is hanging sometimes. my movement is fine and strength is fine. i do a lot of boxing and this pain comes an goes it makes it hard to keep my arm up or throw punches. not painful but dull ache?

You should halt boxing for a couple weeks until that symptoms resolve completely.

If you have tenderness at the bottom tip of your deltoid muscle,if there's pain there if you stand with the arm hanging & then lift it away from your torso or pain on reaching behind your back, I'd suspect inflammation of the biceps tendon. Rest & a course of anti-inflammatory meds like ibuprofen or naproxen are advisable. Or it could be simply an overuse syndrome requiring rest from boxing.