Left mid back when pressed hard feel sort of a tender swollen spot, no pain in back or side, could this be a muscle, very nervous!

Back pain: Yes, it could a muscle pain due to a sagging matress, or instance

Very likely you: have experienced some sort of injury or trauma to your back. It is likely a muscle strain/soft tissue injury. This is fairly common and a person often doesn't recall how or when it happened. Often there is nothing to see (no bruising, etc). Ice it for 15 min 3-4'times a day; take ibuprofen according to directions. It should slowly improve; if not see your Doctor. Good wishes:)