Every now and again this happens to me.
I start dropping combos, it's like I can't be bothered, I do terribly in KoH's or against friends, and I put it down to one of two things. Playing too much, or needing a change.
About 2 months ago this started happening to me. Until I decided on a new main. It was either going to be CSZ or Shang, I chose CSZ and now he is easily my favourite, if not best character. A week or so ago I started feeling like this again, so I picked up Quan Chi. He's okay, but feels a bit cheap...
Or maybe you just need a break from the game for a few days.