it's doubtful you could damage a stainless steel plate from a blow that didn't even produce any bruising or swelling. that doesn't mean you didn't aggravate the surrounding tissue or injure a different structure such as a tendon or ligament. the only way to really tell what's going on is to see a health care provider for a physical examination. given your description however you might also choose to take a watch-and-wait attitude. you can try resting the arm (or avoiding those motions that cause pain) to see if the pain clears up. sometimes a little rest is all it takes. if the symptoms don't clear up or if they get worse -- or if by some chance you develop a fever -- then i'd suggest you definitely seek medical attention. any time symptoms go from better to worse instead of vice-versa it's worth a trip to the doctor. hope this helps!
hit in an arm repaired with plates from fractures 10 years ago. it doesn't hurt but aches with weight. plate damaged? . i had 7 fractures in my humerus and it was repaired with a stainless steel plate and 10 screws. the collision was on the side opposite the plate. i was in tremendous pain right after the collision but it mostly subsided within 30 minutes. there is no bruising or swelling i have full range of motion and have no problem lifting things straight up. however there is a somewhat constant ache and pain when i lift to the side. also there's pain if i try to lift a lot (ex. 3 pushups).