Should I be worried about this bruise?

It doesn't sound very concerning and the fact that you mention you are a vegetarian helps. I am familiar with vegetarianism (was vegetarian from ages 16-26) and have experienced what you are describing from time to time. At least you know where your bruise came from - mine I generally refer to as "mystery bruises". :) Anyhow, do you supplement your diet with vitamins of any kind? Being low in certain nutrients can lead to easily being bruised - especially B12 which is often found in meats, fish, dairy and eggs - things that many vegetarians eat little of, depending on their diet choices. As long as the bruise begins to fade soon and doesn't worsen from this point on I think you have nothing to worry about except maybe picking up a good vegetarian multivitamin the next time you're out and about. :) Good luck!