google told me that my site had been cached on:

12 september
15 september
20 september
23 september

now it says 20 september again! why would that be? (i did clear my history and cache etc before checking)

twelfty

It's possible that you are querying a different data centre.

Google's cache often changes this way because of displaying different data centers cache.

Even a rollbakc might have been there on the same data center. You should not be worrying about the dates if you have the traffic almost same as before.