paint gone funny

I took my chevy truck off the shelf today to do some outside pics for the bracket and noticed the paint has gone funny hence no outside pics. The colour seems to be cracking but not the laquer. I tried to take pics but it's not showing up in them, and the primer isn't showing but it's annoying me. It all looked decent for the weeks/months it was painted so must have only happened since I finished it and I'm just wondering what I did wrong. It's only on the blue, but all my paint was the same brand of acrylics(holts duplicolour). It's just sat on an open shelf and not sat in direct sunlight, (I live in scotland, the sun is something to be scarred by, pasty skin and all that, lol) The cracks while not showing primer are about an inch apart but are only on flat areas, such as the sides and hood top, yet the curved areas are still ok.