So, here's what ended up happening.

I tried to attack!
My roll: 11 (+7) = 18
Thief's roll: 13 (+6) = 19

I miss... 
Backstabs tried to attack!
Her roll: 17 (+9) = 26
Thief's roll: 8 (+6) = 14

She hits for... 9 (+9) + 1 - 5  = 14 damage (17 / 50 HP)

I tried to check the thief for stolen items...
My roll: 16 (+ 3 [reduced, due to being tired])
I manage to find something.