What's happened to my truck ??

[70 GMC Trucker]

I parked it for the winter and took it out of the shed today and look what has happened to the paint. It seems to have little bubbles all over the truck, some areas are worse than others. I thought maybe the cover may have something to do with but not all areas seem to be affected. It was painted 3 years ago by PO and I have had it since last April. Any idea's , this makes me sick and I can't stand the thought of a new paint job.