I can't get my jeans to ever fit right?

It’s quite irritating. I’ll try on a pair of jeans in the store. They’ll fit perfectly or maybe just a tad bit tight (but nothing to worry about). A couple weeks later those same jeans will then be too big and start sagging. Even after washing and drying, I can put them on and a few hours later in the day they will start being loose, especially in the butt area. I know I’m one of the few black girls that doesn’t have an ass lol, but it’s not so non existent to the point where all of my jeans should sag and make me look like I have pancake butt :( I wear a size 27 (or size ¾) in most jeans, with the occasional size 2 mixed in. I’m putting this in general because I really don’t know what the issue is and I want some serious answers. I just want some jeans that fit right because lately I’ve been avoiding them due to saggage. I can’t really go a size smaller because then in the store the size smaller won’t even get up my legs or button. I have to do that weird “chicken walk” to get into the pants so I need some way to get my jeans to not stretch because even trying on the smaller size isn’t possible. I’m not losing any weight so I don’t know what the issue is.