Sprockets,

might it be a matter of seat adjustment??

see

Bicycling Magazine's Complete Guide to Bicycle Maintenance and Repair
page 25

Fitting you to the frame

You should be able to comfortably straddle the the top tube of the bike with your feet flat on the ground.

Level the saddle first. Adjust the height of the saddle so you can maintain a slight bend in your knee at the bottom of your pedal stroke with your foot held level.

For the next step you will probably need help. In position on the bike, rotate your crankset backward until your right crankarm points straight ahead. With the crankarm horizontal, a vertical line passing through the pedal axle should intersect your knee about 1/2 inch behind its front surface. Loosen the saddle clamp and slide the saddle forward or back to achieve this ideal position if possible.

With saddle correctly adjusted, your arms locked straight , and hands in position on top of brake lever hoods, your back and the bike's top frame should form an angle equal to or slightly smaller than 45 degrees.