Correcting a pedaling 'knock'

During the cooldown after my last race, I realised I had a really bad knock. My bike would jerk forward under me. I’ve experienced a similar thing at the end of the a TR session on the rollers, but it was very pronounced this time.

I couldn’t pinpoint exactly where in the pedal stroke it occurs, but it was definitely one side only.
Has anyone had an success troubleshooting this kind of problem before? I’m guessing I have a muscle imbalance, but don’t where to start.

Bottom bracket?
Stuck chain link?

Unfortunately not. My previous experience of knock on rollers was on another bike. I can’t blame the bike here…