Dents on the sides of the back (waist line)?

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I noticed dents on the sides of my back,they go from the waistline downwards,i think right behind the obliques. They are getting more noticeable by the day. For the life of me i can't figure out what they are,are they supposed to be there? I've been doing a lot of strength training\lifting so i'm guessing it has to do with some muscle?