My breast was very itchy in the shower. i scratched it and when i looked down, i noticed that i had broken blood vessels. it looks like a giant hickey. i know i probably scratched too hard, but is this a sign of something i should be worried about?

Worrying is not worth while; it is better to check if you have broken vessels any other area because it may mean something then if you did not scratch prior. I would also suggest to check your breast for any lump and follow the recommendation of routine care with probably mammogram. If you have long nails try not to scratch instead rub lotion on itches, or cortaid on insect bite areas.