Thanks for your query.
Did the bruise
develop on its own or do you remember any injury
And things like whether such lesion is only restricted to forearm or any other parts of the body.
And whether any past history of such lesions and any others in your family have such lesions.
And whether any food or exposure to some material is the reason?
All the above things need to be addressed.
So visit a dermatologist
Hope I have answered your query.
I will be available for your follow up queries if any.
Wishing you good health.