I fell down the stairs yesterday and landed square in the middle of my back. My back is stiff today but I woke up last night with stomach pain. It feels worse then a stomach ache but does not hurt to breath. The stomach pain has been happening on and off today as well. Sometimes it s sharp but it feels mainly like a constant bubble of pain in my stomach.
According to your description the impact was on the back. It is very unlikely to cause stomach pain following a trauma with the impact being back. So if you did not have any direct impact over your stomach and also if you don't have any deep pain while touching your stomach just ignore your pain and take some paracetamol. If pain persist consider other possibilities.