Verified answer

Possible some crap got dislodged from the system and is clogging the shower head?

First, I'd take it off and clean it and see what I get.

Then I call the guy who did the job and ask him

Others are right -- any time you do work on older plumbing you end up dislodging minerals that have built up inside the pipe walls. Once you turn the water back on that crud flows to the faucet and shower heads and clogs up behind the spout. Just twist the shower head off the pipe stem and clean it out in the sink. you will likely find bits of what look like sand or gravel trapped behind it. While it is off, not a bad idea to soak the shower head overnight in a jar or bowl of vinegar. This will dissolve any minerals trapped in the head itself and improve water flow. I just did that with my shower head -- quite an improvement.

I would call back the plumber and ask him to re-check his work he may not have reconnected the pipes correctly.

pull the shower head and clean, that'll fix it!