I'm fed up of my bikini style knickers creating a cheesewire effect on my less-than-firm bottom, so you can see lines and lumps through clothes. I'm generally slim but carry weight on my bum, so losing weight isn't an option. I think I need to size up, but I'm also considering a new style - shorts? midi? full briefs? What and where do you buy? They need to be cotton or cotton rich. Thanks very much.