"Overthought it" can mean several things:
1. Tess got too fat to wear her party dresses and lost her shit
2. This "friend" is one of those paid black women, and Tess didn't sit on enough cakes this week to afford her
3. Tess found out no celebrities were attending and decided it's not worth her time
4. Tess couldn't trick Bowie into smelling the chloroform soaked rag
5. As
@NeoGAF Lurker mentioned, Tess found out Ollie or someone else who bruised her ego is also going.
Tess never passes on a chance to out to show everyone what a glam and popular person she is - there is no way she would spend all that time getting her face beat and then bail at the last minute.
Or she's just lying. She expected to be invited to a party, got all worked up, was told "lol you're not invited", and made that video so she can let everyone know how she had better things to do than go to some dumb party, like sitting at home watching Netflix!