I wouldn't move the saddle, especially since you said your lbs did a good job fitting you. You may still gain flexibility and core strength, which may solve the problem. If it continues, it could be a reach issue, which is fixed by handlebar/stem adjustments (you slide forward as your pedaling because the reach is too long, then push pack to be in the right place for your lower body). Moving the saddle will change the relationship of your knee over pedal, which you don't want to do if it's in the right place already. I hope this makes sense!
Of course, it could be the saddle tilt, too. Or a bunch of other things. Welcome to the wonderful world of cycling, and all it's mysteries!
'02 Eddy Merckx Fuga, Selle An Atomica
'85 Eddy Merckx Professional, Selle An Atomica
'10 Soma Double Cross DC, Selle An Atomica
Slacker on wheels.