That's where i would start, you need the pads back out to have a good look. Wheel back off, undo the 2 big bolts that hold the caliper onto the hub. If it does not come free easily lever it off the disc.
then take the pads back out, and have a good look at the caliper pistons (be sure not to let the caliper dangle on the brakeline - support it).
can you push the pistons back so they are level with the caliper (or at least even on both sides)? If not get the caliper rebuilt (or a new one if it is a G4/D2/Ksport one lol)
if so push them right back, and put the caliper back on the disc.
is the disc exactly in the centre of the caliper? If not, do you have the correct offset disc? O if it is close to centred you can use a washer or shim to space the caliper inwards a little - put the washer between the hub and the caliper.