My now 14 yr old daughter had one on her arm... it first started when she was a couple of months old, it looked just like a small blood blister and it then just got bigger and more raised over the next few months and ended up the size of a 20 c peice and was always very warm to touch.
I got the stares and comments too. One lady even asked me what "I "did to my daughter so I would always put her in sleeved tops to cover it cause it just made me so angry.
I comtemplated having it lasered off but thankfully it disapeared by the time she started school.