What Causes Discolouration On Leg And Purple Joints In An Infant?

Hello, Our son was a TOF baby at birth and has been home now for 2 months, weight gain is fine and he is generally healthy. Tonight he has woken up with a cry we haven't heard and has a discoloration on his legs, they are quite red with a purple in the joints. I first thought circulation but there was nothing impeding his blood flow i could see. Any idea what it could be?