Have you tried feeding her in a different position on that side (ie football hold)? Just thinking it might not be the routine at all, but something which is causing her to be uncomfortable feeding on that side

Hmmm...just re-read your post and realised she's getting worked up before her feed, not at the start of it.

All babies generally have one side they prefer, usually where the flow is better/faster/easier - perhaps she's that worked up that she just wants her 'easy' side asap??