bitter peppers

Can anyone tell me why my (sweet) peppers are so bitter? They were started off indoors, then moved into a low polytunnel/cloche until the hot weather set in, when I decided I might as well take the polythene off until the weather cooled down. Unfortunately by the time that happened the polythene had disintegrated  (I thought I had bought the same polythene I got last year which lasted all summer, but apparently not.) So most of the ripening happened during the cooler weather, or brought home to ripen indoors. They taste fairly bitter raw, but even worse when cooked. Any ideas?