You dont have to be female or pregnant to get stretchmarks. My boyfriend has stretch marks. My brother has stretchmarks. They start off as red marks or lines then over the years tend to fade into a paler colour. I really really think this has nothing at all to do with your ex girlfriends nails. It's just coincidence. When you're digging your nails into a bloke, the easiest thing to scratch from that angle is the back and hips. This is also one of the most common places for a bloke to get stretchmarks.
It just means you're growing. It doesnt mean your ex was trying to sabotage you by putting poison under her nails. Or that you are allergic to her. Hell, after 6 months, I think the allergy would have died down....
It's really fine