What are the big purple bruises on my arm which normally pretty and come every now then Ive never really seen them anywhere else except arms?

It's important to consult with a medical professional if you have recurring purplish bruises on your arms. They could indicate an underlying health issue that requires attention. Your doctor can accurately diagnose the cause of your bruises and provide the appropriate treatment. Without a proper diagnosis and medical advice, it's difficult to provide specific information or identify what may be causing these bruises.