I Lifted My Fathers 140lb Dog And I Instantly Felt

I lifted my fathers 140lb dog and I instantly felt pain in my lower back.. I'm not sure if I strained or pulled it. it had been hurting for a a lil longer than a week and I noticed this evening I have a indentation on my lower back. it's about 4inches up from the top of my but directly inbetween my two checks. just wondering if you you the why and what