Cleaning the calcium from the shower head with vinegar is the best DIY
remedy. Have you done any other plumbing jobs in the past six
months? Loose debris in the line may be part of the problem, as
well. Flush the shower head, in reverse, to remove any loose debris.
With the head off, run the water (hot & cold) to flush the pipes,
too. Lightly tap on the pipes, if you can, to further dislodge any
debris... . anything visible come out? I suppose your house has some
age to it..... plumbing, too?

Sonny