6 year old gets bruises (look like hes has sucked his arm) in same place near his shoulder every so often. happened 3x now. 1st on left arm. then right. and then again just on left. they come out of nowhere. he cant tell me if hes done something?

Definitely need more info, including a pic of the bruises, more info from the child, & more from the parents. I'm assuming no fever, no other complaints, no meds, no limitation in movement. If it looks like he's sucked his arm, he may be doing that. Or, he may be wearing something or doing something that causes periodic abrasions. Perhaps needs to be watched while playing. Probably not concerning.