I can’t find it either, but maybe somebody else can come up with the link if it’s still around.
It does sound like you are tipping - most likely, IF you are actually sitting up straight, you are probably dragging your pubic bone forward in the ‘scoop’ phase of canter instead of ‘scooping’ with your seat bones. And before somebody jumps on me, by ‘scoop’ I do not mean actually moving back and forth in the saddle seat, just the rotation of the pelvis/SI.
I would also look at how the saddle fits you - are you using a new saddle? Different stirrup length? Is it balanced front to back? Too big or too small, narrow or wide twist? And carefully consider underwear and breeches choices. The usual rec’s for Inverness are things like chamois butter or other anti-chafe products, maybe some padded underwear but it doesn’t sound like that’s what’s ailing you.
In the mean time, if you are suffering from raw spots or other superficial pains that hurt to walk around with, get thee some Dermoplast spray at any drugstore. Use liberally and try not to work on sitting deep again until you’re healed! I wish I’d known about that stuff before having a kid, it would’ve come in handy after some saddles.