What's wrong with my ankle??

So last night at work, I banged my ankle on the corner of this metal thing and it hurt but not like excruciatingly so I just brushed it off. It left a really small bruise and I didnt think anything of it. I went to work this morning, and as my shift went on, it started getting noticeably swollen and the bruise got bigger. I worked 6 hours and I just got home and took off my shoes and socks and it is about 3 times bigger than my other ankle and the bruise is about the size of a golf ball and is noticeably darker. Will it go away? Why did it swell so big if all I did was hit it on something?