Why do I lose my appetite when I think of a certain person?

I’m not sure how to explain this. When I think of this person, I lose my appetite. Which means I haven’t really eaten much all day. I don’t feel like eating chocolate right now, which is extremely rare for me, ‘cause chocolate might be the love of my life. I’ve had this happen before.
Do you have any experiences with this? Why does it happen?

NB! It’s not because this person is disgusting at all.