Your flange bolts probably worked loose, so try tightening them up. Do not over-tighten, or you may crack the base. The fact that the toilet “rocks” indicates, as RP said, that the flange may be a trifle high, or it may mean than the floor has gotten warped (check for leaks at the base). Shims can help in either case. But since the toilet didn’t rock at first, I’d suspect loose flange bolts on a less-than-flat base.
NtP