It probably just your perception of its that’s changing driving on different surfaces for example. I would just check the tyre pressures. You would be able to tell if you had a problem with your power steering. Just keep stationary and turn the wheel with the engine just on tick over then do the same when revving it at about 3, 4 revs. If it lighter to turn when revving it then you may have a problem with your power steering. Also listen out for a wining sound when turning the wheel.