YES! get mad because you don't understand the requirement for 20 champs.
if your team bans 5 of your champs. the enemy bans 5 of your champs.
the enemy picks 5 of your champs, and your 4 teammates pick 4 of your champs,
5+5+5+4 = 19
so you need at least 1 champ to play.
theres a reason to the requirement. as for your particular case, not sure whats going on, but I'd be willing to bet you arent counting right.
are you sure you're not counting the free rotation?
This is what I have come to expect from the players on boards.