I accompanied friend to her DD's Easter play at primary school. Think jam-packed hall with rows of plastic seats and no leg-room. I'm tall so my knees were touching seat in front. We were near the back. Woman in front of me (young mum also with female friend) kept jumping up and clapping throughout performance (nobody else did this, she seemed overexcited every time her DC came on stage though I thought the loud behaviour a bit vulgar and annoying). She also draped her puffa-jacket over chair so it was on my knees though I didn't mind this. What I minded was every time she flung herself back into chair, her chair rammed painfully into my knees! I couldn't move my chair back as mum behind had child on lap. I whispered 'ouch that hurts' a couple of times then 'please be careful with your chair' but she just glared over shoulder at me, so after the 5th knee-slam I immediately jabbed her in the small of her back with my knee (through the gap in chair). She turned round and glared and I whispered 'please stop banging your chair into my legs'. 5 mins later she did it again so again I kneed her in the back again (hard enough to make her jump). She leapt up and started shouting abuse in my face in middle of performance 
Lots of swear words and 'stop kicking me you stupid bitch' etc. I replied 'fuck off and sit down' (not proud of myself for swearing but I was caught off gaurd). She then threatened to 'punch your fucking head in' and I told her to sit down and stop slamming chair into my legs, and her friend then grabbed her, forced her back into seat and the children burst into song so peace was temporarily restored. She didn't stand up again but kept scowling at me.
My friend says she is a bitchy mum nobody likes and not to worry about it. But I feel I also behaved badly.
AIBU to think I was justified to react? How would you have handled this?