Fat loss generally begins to happen quickly then tapers off the longer you do it. First of all, this is normal. Squats become relatively easier as you get better at them. This will help keep your blood sugar level at a reasonably constant rate. Eat beforehand, or don't eat beforehand.
Some people say that doing squats with anything in their tummy is barf city.
Figure out what works for you. Have a couple of antacids before the workout. This seems to work for some people. It is the first place you should look to determine why an exercise doesn't feel right. If after following my suggestions below, the exercise still hurts, don't do it.