I can't be your valentine for medical reasons.

Girl: "I can't be your valentine for medical reasons."
Boy: "Really?"
Girl: "Yeah, you make me sick!"

This joke is mainly a put-down, however it also contains a pun. Sick means to be affected by a medical illness, but it also means to want to vomit. When someone says that something "makes them sick", it means that they do not like the thing to such a large extent that it makes them want to vomit.

The girl here is saying that she doesn't like the boy in such a way that it makes her want to vomit.