Israh and Vaqas 1 very rightly detected the fault. Your car is draging towards the side with better braking than other. For perfect braking, both sides must produce the same amount of friction when brakes are applied. Get the lines, callipers, disk pads and rotor disks checked on both sides. If disk pads are the culprit, make sure your mechanic replaces these on both sides or you may experiece the same problem; this time on the other side.