What Causes Bruise In Arm That Itches And Hurts When Scratched?

i have have had this bruise looking thing on my arm for a while it comes and goes. Right now it looks like a bruise it's about 3 fingers long but it dose not hurt. sometimes if i wear long sleeve shirts it itches and when i scratch at it it will start to hurt and swell. i have been on the birth control shot deprovera for over a year now and a side effect is blood clots . i was never worried until just now when i noticed this bruise. I just want to know what it is.