Learning to draw anatomy and never forgetting it takes a certain finesse. Do you draw from reference, or have you ever been to a life drawing class? Learning the actual fundamentals of anatomy and how it works may prevent anything like that from happening again - however, it won't mean anything if you don't practice, practice, practice.
Of course, stress/anxiety plays a role too. You're not necessarily "forgetting" how to draw, it's just your mind is so clustered with other things that it might make being creative/skillful with a pencil a lot harder than it should be (at least to you). And in turn because you know you're not going to be able to draw right after a big event, you end up psyching yourself out.
I remember one time for a few days I had total art block. I just couldn't seem to draw, and when I did, it came out looking awful and I just wasn't happy about it. I couldn't pinpoint what was wrong.
Then I realized I had been wearing a band-aid on my index finger for the same past few days and it was legitimately fucking up how I held the pen and therefore fucking up the way I drew - what I thought was a mental block was just a stupid band-aid. I took it off, and didn't have a problem again.
This isn't really the same as a band-aid, per se, but when big events happen in your life, whether they be positive or negative, your mind can be swamped with a whole bunch of different distracting feelings, from depression to sadness to anxiety to excitement (the kind of excitement where you're totally overstimulated and can't even think straight).
You might just have to take a day off to relax, clear your head, have some you time. Then when you're all clear and ready to go, try it again later.
Hope that helps!
