It could be your suspension, You might want to find a level area like your garage floor and put jack stands raised equally under the 4 corners and see if they all settle evenly. You might also take a plumb bob and make a spot on the floor at each corner then take a measurement from corner to corner (in an X) to see if the frame is out of square. If you have any discrepancy the best thing is to let a good quality frame shop push and pull it back to straight. If it's your suspension start shimming and replacing. Or it could level out when you are driving depending how heavy you are.