June 1st we moved into a 4th (top) floor apartment in an Art Deco-era building in Denver, CO. It came with a claw-foot tub with a shower that was a simple piece of pipe with a shower head. The shower head only came up to my forehead, so at the end of June I installed an 8" extension/coupler.

Everything was great for a couple of weeks, but now when you are showering, not while bathing, the cold water will increase pretty quickly. It even turns the knob. Weird? I noticed that this only happens when you pull that pin that directs the water (and pressure) up the shower pipe. There was no problem initially. I had replaced the shower head as well when extending the pipe, but it was cheap and sprung a had a pinhole leak at the fitting. After I reinstalled the original shower head this weird water pressure thing popped up.

Any ideas?