This happened to my husband too - but he wasn't on Revastigmine.
It panicked me the first time it happened, I thought he had had some kind of mini stroke and I was straight on the phone to the doctor. I was told that it is common for this 'lean' to develop as the disease progresses as there is a gradual loss of core body strength.
Initially the leaning would occur only as he tired. For instance we could set out on a walk just fine but the lean would start to appear towards the end of a walk, so I started to make our walks shorter to allow for this.
Eventually though, he would lean to one side even when sitting in his armchair and I would help support his posture with strategically placed cushions. He remained mobile (though increasingly slower) for a number of years and only actually lost mobility at the very end.
I have heard that constipation can also cause this 'lean' to appear at times.
Definitely let the doctor know and see what he/she advises.