Why did i break out into a rash after eating chocolate?

around ten minutes ago, i ate a couple of mounds bars. a few minutes ago, i started to break out into what looks like a rash. theyre little red bumps on my neck and right below my neck on my upper chest area and they itch. im wondering why this happened, because i recently ate chocolate and ive had it before and this has never happened before to me when i ate it. did i suddenly become allergic? and will the rash go away? has this happened to anyone else before? thanks for all answers