Did you completely take the knife apart? If so, one possibility is that the pivot is ever so less tight as it originally was, BUT it feels equally tight as before because the washers may not be on the original sides/angle, and this can result in exerting more resistance to the blade's ability to swing/swing-freely for a short while as the washers slightly wear and smooth out accordingly to the surfaces they are making contact with (which are harder than the bronze washers). While this happens on a very minor level, it sometimes can make enough difference to be noticed. I am describing this absolutely terribly, but the best comparison I can make this to is how it is advised that after installing a new wheel on a car that the lug nuts be slightly tightened after a short period of usage to adjust for the very minor initial wear on parts than can occur shortly after a change is made. The solution to this is to more or less either use the knife as-is and check a few times to see if the pivot needs to be tightened, or (my preference) to tighten the pivot to a point where the resistance it applies to the blade is slightly greater than it was before and if this is the issue, the resistance the blade has will decrease slightly even though the pivot screw itself did not move.
Another cause of blade play can be when the standoff or spacer screws (i.e., the screws other than those on the pivot and pocket clip) are not tightened evenly, and in some cases are over-tightened.
Also, what lubricant did you use? The type of lubricant can affect how much resistance the blade sees quite a bit, which can in-turn affect how much the user tightens the pivot.. For example, the lubricant I use on the pivot/washers/omega springs/AXIS contact points on my Benchmades results in a blade that swings completely freely versus how most Benchmades do not swing freely from the factory, but with much smoother operation and yet the pivot itself is about equally tight as it was from the factory.
Finally, is your blade coated?
