It's an ancient bug that sometimes you can't see your own health or ammo (or health and ammo) in your hud. For medics the same can happen with uber percentage.
(example: https://www.youtube.com/watch?v=-xJyNDJkceQ)

The way to fix this is just inspect another players loadout
command: +inspect
Default key is F

Sorry if a thread has already been made for this. I just wanted to get it out there because a lot of players don't seem to know this yet.