I hope it is a single lesion and you do not have similar lesions anywhere else in your body.
It can be anything from a bug bite
to allergic response
to exogenous substances to bruit due to unnoticed trauma.
Stay calm; apply ice cold
packs to the affected area thrice daily for 2 days.
Take Tablet Zyrtec
10mg at bedtime after dinner for 2 days.
You should be fine soon.