Moves side to side as in freely wit a hard limit to this movement that is not brake rotor related? Then the first I think of is a loose hub bearing. With this the hub shell, and the spokes/rim unit, will slop about the axle. So if the axle is securely in the drop outs then this is a common point of movement. Removing the wheel from the fork and holding the axle in hand then wiggling it can often produce the same movement, al be it to a lesser degree as the axle spans a smaller distance from bearing to bearing then the rim is from the bearings.


Other possibilities are a not fully tightened axle in fork, the QR is loose. Or a really worn our fork bushing. Extremely loose spokes. A headset that's really loose and the not seeing the real movement. Andy