It is possible the mega is not registering a High logic level at 3.3V , ideally you want 5V input High level. Otherwise it can be caused by long wires or improper grounds. I am not sure why you can't use the same pin for both, I'm sure the current won't be exceeded. You could even power the ph module from an output pin set to high, I am sure that the 14mA current is within limits, but you should look it up.