Stain problem

[tintin]

I have a purple stain on a white t-shirt of mine. Thing is when I was wearing it I had a jumper over it and am completely mythed where it's come from...

Must have been internal.

I remember being out with some friends, and one suddenly had an itch in his left armpit.
Eventually he took his shirt off and found a hole, around 2 inches across, burnt in it.

Still a mystery.