Why did I completely blunder this?

You might want to post your game here, dude. Why make people do the extra work when you're asking for help? Or, if you don't know, like you said - maybe learn to do that first.

In this thread good man Blueemu explains in details how to post our games here.
"I understand why I lost so many games lately". - Chess Forums - Chess.com

Stop to make so many blunders. That's all.
The secret is very simple.
I was black, obviously. Sorry not for showing the game immediatly, I was on my phone when creating the question.
You apparently were never in a "completely winning position" in the RRN vs Q endgame. You lost it when you blundered your knight.

It's virtually unwinnable for black after the knight is lost. White has a lot going for him. The white king is safe and your king is in the middle. White has passed pawns, 3 of them being connected. White is going to try to promote a pawn, but you will have a lot a trouble taking that pawn while avoiding forks.