Sudden arrival of left hook

I have always used the same setup, grip etc and hit the ball straight. however, over the last few weeks a pull to the left has arrived in my iron play. I check on a square set up etc and seems to be ok, but I cant come to a conclusion on why this has set into my game. Any advice appreciated.
regards,
Robert

more info please..... with long irons/ short, mid

you also said a pull has arived, but title is hook, which is it... or is it a combo?

If you are certain you did not strengthen your grip, then it'd be interesting to find out if you have been working on certain things at impact, like rotating forearms perhaps?

Sorry for confusing you. What I mean is shots going to the left, with a square adress.
Thanks,
Robert

When this happens I try to get the feeling of letting the right elbow lead the club into impact....it tends to get the ball back starting on line.

1) Pull - straight left is an outside to inside swing with a square club face relative to the swing path.
2) Pull hook - outside to inside swing with a closed club face
3) Hook - an exaggerated inside to outside swing with a closed club face

If 1 or 2, try hitting the inside quarter of the ball. For visual, place a shoe box 1-2 inches outside the target line in front of the ball

If 3 - following though more and hold the follow through. Make sure you turn your upper body competely through the shot.