What causes a rainbow shark to develop a bulge in belly,now not swimming right after a partial water change?

It's coloring is good but breathing is more rapid. No other sharks in tank. It hid for three days after first acting abnormal when a partial water change was done. Thought it was dead, but now it has reappeared and acts like it's belly is waiting it down and making it so he can't hardly swim. It seems to try and roll over and trys to balance itself on objects in tank.