my sheild just randomly stopped working and only started to work again when i ate something

I started a new world and about an hour or 2 in I was doing pretty good with armor and all that. I was in a cave and I tried to block a zombie with my shield but my shield didn't go down when I hit shift, tried again and the same thing kept happening. When I hit "e" before I uncrouched the shield was in my head. I thought this meant I could still block so when I creeper came I tried to but got blown to bits. Went back for my stuff and it was still broken until I ate something then it started going down again. I didn't have any mods or anything and I also have no footage of it either.