Hi Roz,
That looks very much like a subcutaneous bleed, they are common in people on steroids and some other meds which cause considerable fragility of the tiny vessels just under the skin. Sometimes even a tight sleeve or a softy knock can cause this to happen.
I do know that subcutaneous bleeding can be a symptom of RSD--I wonder whether it is the disease or the treatment?

It is one of those "chicken or the egg" scenarios.
I do know that much care needs to be taken to prevent tearing of the skin above the haematoma as a non healing wound may well be the horrible outcome (speaking from experience here)
Please take care, I guess wrapping yourself in cotton wool is out of the question??I feel as though we do need to do that sometimes
Please look after yourself
Tayla