Why do i always get random tiny red spots under the skin on my legs?

but they get lost in a few days but then another will appear, its like tiny red spots together in one spot. but theyll disappear but another will come later on. my dad says its how i always scratch it or how maybe how i always hit my leg with the tables thats why its like a bruise inside. but i only get those on my legs, never in my arms or anything.