What should I do if my arm is bruised while playing volleyball? There are always some problems like this.

If it is the cushion part (forearm, palm side), it is generally because it has not been practiced for a long time. Once you practice the cushion, the capillaries will burst and there will be blood spots, which may look like bruises, but if you keep practicing, there will be no more. Stop practicing for a while and it will appear again.

If it was somewhere else, did you accidentally hit it? If the opponent hits the ball hard, and you are sensitive and prone to subcutaneous bleeding, there may be bruises, but there is no other discomfort, and you will soon be able to absorb it yourself.