Can anyone tell why my silk and matt and gloss paint turns to curd in my shed?

hi..every time I go for a tin of paint out of my shed it looks like curd;I can buy a tin of new paint and after a couple of month it will be like curd I only use a little out of a big tin then make sure the lid is on tight but it still turns to curd..sometimes the tin of paint is solid not just the top part but the whole tin.. and no amount of stirring will change it..my garden is south facing and gets very hot but it has even gone like that in the winter any one any ideas.I now keep it in the house I just wondered if there are any decorators out there who know why the paint goes off