So, if you run the cold water in the shower, it has plenty of pressure?
As stated by majorloser, if you have a modern place or was retrofitted to comply with new codes, the shower would have a pressure balance system of some kind so when you flush the commode, the shower will not sculled you. I have one and it is the hot side that seems to be affected, for some reason, by the mechanism in the mixing valve. Mine is a Delta and uses a stainless steel piston that balances pressure and over time it has a buildup that affects the hot side travel. I bet that is what your issue is.
I take mine apart and use 400 wet/dry sandpaper or finer and sand the piston. Good for another block of time
If it was calcium build up, you would have the issue at other faucets too, not just the shower head.