It is normal in a way. I've gone through the same thing many times, learning a fast lead... being able to play it... then a few months later it's like my brain doesn't want to work anymore.

This might not be the advice you want to hear, but this is the time when you should be slowing down, playing to either a metronome or preferably a drum machine, and working on your accuracy and cleanliness.
Playing fast is fine, but if it's sloppy and doesn't have any feel to it, nobody wants to listen to it.

Try playing leads to a drum beat and slowly speed it up every day.
You could also try working on things like economy picking or starting with an upstroke instead of a downstroke just to shake things up a bit.
One of your problems might just be boredom from doing the same thing all the time.

No matter how much I spend rehearsing something I always leave some time to just go nuts and get into playing something I like. Otherwise it just seems like work instead of fun.