Bike might be a little big. Saddle might be a little high. Try this: with your shoes on, put your heel on the pedal. Rotate the pedals until the pedal with the heel is at bottom, aligned with the seat tube. With your knee completely locked out and no hip rocking, your heel should barely stay in contact with the pedal, or perhaps have up to a 4mm gap, no more.

Check that your saddle is level. A little pushing back from time to time is normal, but the spot you gravitate to should be perfectly comfortable. Try rotating your pelvis forward a little, putting your sit bones in firmer contact with the saddle. Bend your elbows about 15° and see what that does. You shouldn't ride with your arms straight like that - nothing to absorb road shock and too much torso showing to the wind. To check stem length with respect to saddle position, with hands on the hoods, bend your elbows to 90° while pedaling. Your elbows should be about 2" directly in front of your knees.