They don't sound like much. My guess is that in each place there was mild irritation of some kind, very possibly from hitting the area as you yourself suggest, that left behind pigmentation. In any case, you are not going to be able to diagnose this on the internet. Since the spots worry you, I suggest you see a skin doctor. It should take one visit to clarify the matter, and then you can most likely forget about it.
Best.
Dr. Rockoff