That's regular with adsense. Sometimes some higher paying ads display on your pages, when clicked they get you more cents per click. Other times it's low paying ads.
For the last few days, my income has fallen like 50% and I am getting almost double clicks I used to get earlier... It's just that now google is paying way less per click as I used to get.
Concentrate on increasing your click rate instead of how much you get paid per click. Do some optimization!
Bye
