Why is it that I used to be able to spell very well & use EXCELLENT grammar, but now I'm losing that sense?

It's like for the past 8 months or so, I've been having to question (to myself), how to spell certain words or question (to myself) if my grammar is being incorrect. I never had any spelling/grammar issues before. I'm beginning to wonder if my brain cells are dying. I've also been very forgetful lately, too. I'm 24 BTW.