I've never had/bred guppies, so will defer to those with more experience than yours truly if they see it differently, but I personally wouldn't be to worried.
I think it'd be pretty normal to show signs of stress (clamped fins) immediately after giving birth. I'd keep an eye on the fish and look out for any additional changes, and keep an eye on water quality (which you're doing), but again, showing signs of stress after giving birth would be, I think, pretty normal under the circumstances.
Good luck!