That sounds like a bug. Were you by any chance using Shadowbound with 1 turn left and 0 health when you beat her? I think that might be the reason. Normally after you beat her and she drinks those health potions you're supposed to be brought out of combat and talk to her again, (not get a game over) at which point she'll send her royal guard after you. It's pretty hard to beat the royal guard on low magicka, but spamming shadowbound and upgraded fury of Lavanya makes short work of them (they cannot silence you as Raa does, and they will drop a strong weapon if defeated), so if it works to beat her this time, just go inside her catacombs after winning the fight with Raa and rest at the arcane fire before you come back to talk to her so that you have full mp and hp for the battle against the guards! About the original bug I can't think of any other fixes except:
1. make sure you've got either some health left or shadowbound cast with at least 2 turns left when you beat her, or
2. maybe check that you've got the latest version?
I just finished the current version of the game myself after ignoring it for years due to people complaining about bugs, only to find out it still has unsolved bugs even after not getting an update in ages! That's just the Domina experience for you - dominating the crap out of good coding ever since 2018!