I just had a huge argument with my mum throwaway account
Why would you get into an argument with your mom while on a fake account?
She was upset I brought my girlfriend home.
It's understandable to be upset when you're not allowed to bring your girlfriend over, however its wrong to deceive others online by pretending to be someone else. If you're upset you shouldn't be passive aggressive and should just talk the issue over with your mom. If she finds out it was you arguing with her online, she'd be even more upset.
I pretended to be her husband and she found out I wasn't but doesn't know my full identity.
I don't get how you might think she wouldn't find out. Does she not talk with her husband ever? Maybe you should step away from social media for a while and keep your head down, so you don't cause more trouble.