Hickeys are pretty much ways of marking your territory. I refuse to let women kiss my neck for longer than a split second out of fear they're going to give me one of these, personally. Whenever I see one, that typically denotes to me that whoever the person is, they've probably been making out with a teenager...
Anyway, this reaction isn't surprising if she works in a job where she's in front of the public all the time and has regular male patrons whom she flirts with for tips. Hickeys get in the way of that.
Additionally, as it's a casual relationship, bear in mind that there's a good chance any other partners of hers are going to see the hickey and think, "Eww, gross," so you're affecting her life in negative ways with this. All in all, it's a somewhat juvenile way of warning off other men from a girl, and women know what it is and tend to view men who give them to them as possessive and jealous. Ultimately, not very attractive traits in a casual partner...
I'd suggest cutting this one out, or sticking to giving her hickeys in hard-to-see places like the back of her knee if you absolutely must give her one, so as not to interfere with her work or her other relationships.
Chase
