Screen shot no longer working

I do a lot of screen shots using CmdShift4.

For no apparent reason, CmdShift4 now captures the whole screen, while turning everything blue and displaying a tiny camera icon. I can't drag a marquee to capture part of it. I can only click and capture the whole screen. I've never seen this blue phenomenon before.

CmdShift3 now does nothing at all.

I'm running High Sierra 10.13.6 on a MacBook Pro 2018.

I have

-restarted

-zapped the PRAM while restarting

-cleared the screenshot preferences file

-restarted again

Any help would be much appreciated.