mmm...6lbs. per gallon of water.. 24lbs..
well it's possible, i've lifted a bag off school books and pulled a back muslce really good. ggreg has the right idea..probably is a muscle and not a organ. Only organs over there is kidney/colon/pancrease/stomach/gallbladder and I just had a friend had hers removed (gallbladder) because she lifted some heavy tools. You probably pulled something and hopefully it's not a hernia involving the stomach/gallbladder. Probably need imaging to check, a good sign would be GERD or acid reflux happening.
You may also have a hernia, which a family doc can refer you to a specialist. That's where you lift something heavy and your abdominal muscles tear, and it can be quite painful and sometimes needs operating. But yes, there is a muscle that goes from the arm over to the chest, in the location you are experiencing pain. I'm older, and whenever I drag in especially heavy groceries, I'll pull my arm and the side of my chest and it'll hurt for a few days. You can look online at an anatomy illustration of where the muscles are and how they connect. Also, there are nerves that go from the back to around the chest at the thoracic spine level, altho you said your lower back was hurting. I personally think the four gallons isn't enough to really goof you up permanently, but those would be the things I would think of from someone who is in pain after lifting something heavy.