Triton Amber II is leaking from overflow pipe?

My Triton Amber II shower worked perfectly fine for a few weeks, then all of a sudden when I went to use it cold water started coming out of the overflow pipe. When i switch the shower off then the water from overflow stops as well. The only thing different I have done to the shower is fit a new head which has the option for a more forceful spray. I have tried taking the shower head off incase that was the reason but even with it removed it still did the same. Am desperate for a lovely soothing shower please help :(