Why does my coworker touch me all of the time?

She has been dating someone on and off for a so time now but keeps touching me. She is in her mid 20's and I'm 20 almost 21? She rubs my back and winks at me and one time pinched my nipple. When I'm in her way she hip checks me instead of asking me to move. Today she snuck behind me and poked me in the back with her fingernails repeatedly for like 5 seconds. She told as she rubbed my back that she was going make kick like a dog and asked if I would like that. Its really creepy and im actually like another girl who I'm sort of dating on the down low who really can't say anything about it cause we dont want people to know especially our coworkers.
Why does my coworker touch me all of the time?
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