Stretch Marks?

Kinsey
Every woman in my family has been blessed/cursed with a big butt and thighs. My butt is a full 4 dress sizes bigger than my waist. I do work out. My waist is a size 2 while my butt is a 6. 
I'm getting married soon and I recently noticed what looks like a purple stretch mark on the side of my thigh. I've always worried about my butt and thighs but not too much. My fiancé likes my butt and has never shamed me for anything. However, my insecurity has skyrocketed since discovering this. 
Any advice?