40-10 is a 4.00 K/D but whatever.

If you don't play regularly then you won't be as good as you can be.
If you take a 3 day break don't expect to go back on and average a 1.90 K/D or whatever.
I usually play 5-6 hours a night and I recently took a 3 days break but when I went on I was better than usual, I was playing on a new account and was getting high K/Ds, my K/D is 2.05 on my old account and 2.30 on my new account.
Just sweat it out, go try harding and you should be fine.
If that doesn't help and you really want to get better try investing in a headset and maybe a scuf controller.

I have this problem sometimes. I can go from playing like a boss getting things like 48/8 then the next game getting 20/17. What I do is stop playing for a bit, and I find if i'm getting annoyed about doing bad I'll play even worse so I stop playing for a bit then play again later and I will do well again.

Also if you are playing different games it probably will effect the way you play, for example you said you can't shoot people after playing zombies, that's probably because zombies don't run as fast as people do online.

Just stick to playing online and you will get used to it again, and if you continue playing bad, instead of going for K/D try playing the objective more, maybe you'll rank up more often than going for kills, that's unless you play TDM of course.

Get a couple of team mates to play with everyday, and learn how to callout.