Cheers for the advice. It wont be an old injury because I havent broken anything before.

I think it might be one of two things. I find it hard to make a tight fist with my left hand, so I am punching with a slightly open hand. I think i will change my hand wrap technique with less wrapping around the palms.

Or

When I get tired I think my I am positioning my fist slightly down, rather than parallel to the floor. Whatever it is, I will keep practicing left hooks on the heavybag to try and perfect my technique.