Cramps in back

So as a bigger girl I’ve been starting to work out. But after 5-10 minutes of walking my lower back starts to cramp (like a Charlie horse) if I continue to walk it gets worse and worse and starts moving down the outside of my hips. The good thing is it goes away after 15 minutes of stopping. Does anyone know if this is because I’m fat, or is something wrong? Has this happened to anyone of you?