wow. you just described my exact birthmark except my PWS extends to my scalp.
Yes radomn bleeding happens sometimes. i kinda dribble when i talk for a while. kinda embrassing.
If i am doing something very phyically demanding in the sun i can feel my birthmark pounding but yes the heat does affect it.
I try to take it one day at a time an not think about it too much. denial can work great sometimes LOL