The "bulge" you are speaking of is probably the vent (which is the poop hole to be blunt) which all chameleons have. Many people confuse that with the bulge that the males have. In some of the younger chameleons they will not have the bulge yet so you have to go by the tale base thickness, which can be tricky sometimes. As they get older, if the bulge is not obvious, there are other ways to determine sex, such as with colors, or their rostral size(the thing that sticks out on their nose) The females are usually smaller than the males as they begin to mature.