Mana pool gone

So i was in underot with a q, and on unbound abom, suddenly i cast like 5 spells when right at the start of the fight, try after try after try and i noticed my mana pool only had a cap of 20k and after those spells i used i would have no mana
long story short i cant find anyone with a similar issue to tell me what was happening but it was very frustrating trying to complete a mythic and at the very end im screwed over by a bug… plz fix blizzard or give me mythic keystone gear that i missed out this week because of this