Alarm not working... sometimes

I'm a light sleeper but I'm paranoid about sleeping in so I always set three alarms just in case. The first time I slept in was on the day of a test, I blamed it on uni from home and not having a good sleep schedule even though it had never happened to me before. Since then it has happened some other times but like once every two months, so I always thought something was wrong with me this year.

On Monday I slept in again and now I'm paranoid, I wake up every 40 minutes and guess what, today I was awake before any of my alarms and NONE OF THEM MADE A SOUND.

Thank goodness I also started setting my tablet alarm because otherwise I wouldn't have woken up today. Has anybody experienced this?

Edit: I just set an alarm to ring in one minute and then when I pressed OK suddenly three notifications indicating the three missed alarms appeared, it's as if the app was sleeping or something.