What Causes Rash On Forearm After Taking Plan B?

I took the planb pill around 11am this morning. i was in the shower around 8:45pm and i nopticed a rash on my forearm i know it wasnt there aerly throughout thr day, i didnt put much thought into what it was until my boyfriend said if it was the pill. and i came across this and im glad i am not the only one with this side effect. luckily it isnt horrible. but it still worries me. what should i do