Where did this bruise come from?

[JorisBonson]

Noticed it on Friday and it's just got worse. I have absolutely no idea what I could have done to do this. It's trailing up to my armpit and my bicep hurts.

Any ideas?

(I am normally that veiny yes!)

Caught it in a door?

I’ve had a bruise like that from a too-heavy bag falling off my shoulder and catching it on my arm. Also from some far too enthusiastic dancing at a ceilidh!

None of those things (I wish it was from a ceilidh!). It's a mystery.

A better picture... You can see the bruise going up my arm in this.