Sometimes i get red splotches on my legs that look like a pool of blood is underneath my skin- they're about size of pencil erasers..what is this??

This may be a form: of pigmentary purpura. But there are other causes of vascular trauma. I would see dermatologsit to get correct diagnosis and treatment options. The fact that it comes and goes means you need to see someone when rash is present. Hope that helps

Bruising: If I understood your problem right sound like you are having bruises, what is called echymosis that is bleeding under neath the skin. If happens seldom and as isolated lesion It could be from unnoticed injuries. If it happens to be frequent and occurs in clusters and of larger sizes have it checked out by your physician to rule out bleeding disorders.