My reading buddy has a birthmark/bruise?

I'm in grade eight, and was reading with my reading buddy in SK today. It was hot and she took of her shrug to reveal a huge bump on her lower shoulder. It was a little bigger than a golf ball in size, and raised about 1-2cm above the rest of her arm. It was purple (which is what makes me wonder if its just a HUGE bruise). It also had little red dots all over it. It didn't seem to phase her at all, but I didn't want to ask. I'm just really curious to know what it is.