Rengar ferocity bug

I played a game with Rengar and i had 4 stacks. I jumped on an enemy and mid-air i pressed my "E" that hit the enemy I was jumping to. Then when I was on the ground I tryed to use my empowered "E" like 5 times but it didn't do anything. Then I looked at my skillbar and I saw that empowered "E" was on cooldown... In the rest of the game it never happened again, but if it would happen again I will report again. Sorry for bad english -Edit: it happened again, I had 3 stacks and I used "Q" and "E" in air, then i couldn't use empowered "E"