Increasing Motion in Upper Tricep?

I’m not sure if this is from an injury or not, but whenever I try to do Straight Bridges, my left upper/inner tricep gets really tight after 2-3 reps and feels like it’s cramping up on me. I do them on a low bench because I can barely get 1 off the floor because of my arm.

I believe the muscle that I feel it is either the caput longum or the brachii. I did have tendonitus in my left forearm about two years ago from deadlifting too much, and it would occasionally cause my arm to lock up when extending it with weight (such as when getting up from the floor like Straight Bridges), so maybe it could have something to do with the posterior cutaneous nerve?

Since I cut back from the high volume deading, I haven’t had an issue or even much pain for about a year now and I don’t feel this sensation when doing other exercises such a benching or even dips (assisted).

Does it sound like an injury or is it just weakness? In either case, what can I really do?