What should I do with KK here?

[Baby Mantis]

I am a newbie as well and this is above my limits, but I might would put him on a flush draw. Say AK of diamonds, or he could also be holding AA, although he might would of raised pf with AA. I think his most likely holding is a flush draw probably the nuts. It could be an absolute bluff, I myself would probably call and lose my stack when he turns trips over or catches the flush by the river. That is why I am still small stakes though, can't make the big laydowns.
What did he have btw?