It seems like you must have injure your muscle on your back while you were working out. As you said that there is a bruise
, this definitely tells a twitch
in muscle at that area. Kindly consult a GP near you and get well soon.Also stop going to gym for a few days until your lowe back heals.A detailed examination is needed in your case to confirm if the internal organs are injured or not.
Take care and have a nice time