Small purple dot on my knee just below a graze (don't know if this has anything to do with it) i used to be itchy now it's just sore?

Needs exam: Could be related to the trauma you mentioned, but can't tell without an exam. Your primary care provider would be the best place to start, the timing of which depends on how longits been there and your symptoms. Hope this helps!

Eval: This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes.

Purple spot: could be a blood blister from the injury. If no resolving within 10 days to 2 weeks the see your doctor for review . Look out for signs of infection, redness, pain, heat-- review early if these are present.