Sensitive or care??

Recently i message back one of my friends who is gal....
Please answer me what reason if a gal sudenly very angry even if it is a joke..
read the following conversation (SMS)

Gal: Y dun u get some rest?

Boy: No, i'm nt sleepy yet...

Gal: But u r sick...

Boy: *jokingly reply* sick oni, not going to die... still alive ma.. hahaha...

Gal: *Angrily reply* hey u!! nt gud to say tat!!

then, i convince her n she say dun ever say such things again to me or i don't friend with u anymore.. i promised her.. after few messages, i asked her back y r u so angry just now?
she reply said i dun like my frens to say like that...

Do u all really think thats the reason she got mad?? Is she being sensitive or care abt me or wat?? i don't know la... haiz... comment ur view to me..