Not enough reach is cramping your back plus bike is really too small by a good bit. On this bike, move your saddle back as far as it goes, roll your hips forward on the saddle and straighten your back until it's a straight line from your shorts waistband to the back of your neck. Then add stem length until your upper arms are at a 90° angle to your straightened back.

Of course you have neck and shoulder pain with that fit and position. Look at the angle of your upper back, just below your neck. It's horizontal, like you were in a full aero tuck with a flat, horizontal back. Except you're not.

Ideally, with hands on the hoods and forearms horizontal, your elbows should be in front of your knees.

All the above said, we do see a lot of pros with bikes as small as yours. They do that to get a lighter, stiffer bike and they get paid to win or place, so they put up with the little things like pain and discomfort that come with the small bike. Choices.