Am I overreacting? Girl coworker can't stop wanting to hanging out with my boyfriend.?

You're definitely not overreacting, it sounds like she's trying to move in on him, so I'd do something fast. Definitely talk to your boyfriend about how the whole situation is making you feel and ask him politely to keep his distance from her. Obviously since they work together they'll be seeing each other, buyt they don't have to like hang out and stuff. He's your boyfriend, not hers, so make sure she knows that. Good luck! =]