Welcome and thanks for posting your query to HCM.
It appears from your problem that you have "Purpuric spot". Purpuric spots appears after any minor or major frictional injury
Due to trauma over the skin, blood collects underneath the skin due to rupture of small blood vessels under the skin. Initially it appears as reddish and then turns purplish in next few days. The complete normal skin status takes few weeks of period.
In your case you might had encountered a frictional trauma leading to this spot. You need not to be worried about this spot if it is not progressing in size. No active intervention is required for the same. It would be changing its color to dark black then greenish and lastly yellowish before fading off in next couple of weeks.
If the spot is increasing or multiple new spots are forming then you you must consult either a physician or a dermatologist
Hope these informations are helpful.
"Wish you good health"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)