I had a similar problem after a weight loss. My formerly pear shaped body no longer filled out the curvy cut of jeans I was accustomed to buying. The good news is that you will be able to wear brands that didn't fit your former body. The bad news is that is takes a LOT of trial and error to find your new favorites.
Doing an alteration like Aziraphale suggests will help in the short term for jeans you already own. I did this as a stop gap measure once I found a really good seamstress. A word of caution, if too much is taken out of the back seam the pocket placement and spacing can look odd. A good seamstress will talk you through this.
When looking for new jeans at retail I suggest trying brands that never worked for you before. Brands that have a straighter cut. I still carry my weight in my thighs, although my butt has slimmed down. Prior to weight loss Ann Taylor curvy and Talbots were my best fits. Now, I have good luck with the Gap Girlfriend cut, KUT from the KLOTH boyfriend, some Banana Republic and Jcrew cuts. Lucky brand also has a good variety of cuts to choose from. You can try several and see what fits your new shape.
Good luck!