Why is the water in the shower pulsating?

You can get a water pressure controlling shower head to solve the problem

Just a thought, do you have adjustable shower heads that can give you a gentle shower or with an easy turn of the dial, ring or whatever, the spray can become a massage shower? If you do have that type of shower, maybe someone has changed the setting and forgot to turn it back, or you might have accidently changed the settings during cleaning.

If it'still a plumbing issue, I have no ideas but I'm pretty sure you would be noticing a similar pulse in the water flow at your sinks as well.

IF you are on a well system & not on a city water system: It very well could be a well pump going out; check to see if there is a problem with other faucets in the house...if so...it's a well pump going out & you will need to call a plumber to install a new pump OR PUMP REGULATOR. Also: SOME shower heads have a small piece of plastic in the part where it attaches to the water pipe coming into the shower area, it may be clogged with residual build up from minerals in the water, if so...remove the small plastic piece & clean it [if it happens often, just remove the plastic piece]. Check all your faucet screens for build up of mineral deposits that tend to slow down your water volume.