It hard to say what's going on with your back... first, I will take you have not injured your back recently. So, here are a few things that could be causing your problem: 1) your head stem is too low and/or too long, also while on this your bike (top tube) may be too long, causing you too reach too long and finally while on this your saddle could be way too causing you to reach down too far (less likely at your age), so if all the spacers on your head tube are on top of the stem like a chimney, remove stem and place all spacers under stem and replace stem. If this doesn't help, if your stem is facing down flip it up, you will need to remove bar and replace. If you are still having issues, check stem length if is over 100mm, get a 80mm with a 5-7* rise. Before you do anything check your saddle height, while pedaling at the bottom of your pedal stroke your knee should still be bent about 10* (if you dropped your heel slight lightly you should easily be able to lock out your leg); 2) pedaling in too hard of a gear, use a gear you can spin a lot easier and not worry about your speed, once you get used to spinning go up one harder gear keeping the same spin speed, (if have a cadence meter try to hold between 85-95 rpm); This is a starting point, there so many other things that need to be taken into consideration. The bike fitter may have given you a race fit which is totally different than a bike fit for the average rider. Not saying at some point in time you can benefit from one. Hope this helps some. Happy and safe riding! Coach JQ