What Causes Purple Bruise On The Abdomen?

I am 22 and about two weeks ago, I noticed a purple bruise-like spot on my abdomen. It isn't tender at all, no injury, and it doesn't itch or anything. It doesn't seem to be fading at all and it's very deep purple colored, about a centimeter diameter. I haven't been doing any exercising lately either. The girls at work say get it checked out but I wanted to look on here first. Any thoughts? Thanks.