What Causes Dull Pulsating Pain In Left Side Of Rib Cage In A Person With Spondylolisthesis?

I have had a dull pulsating pain in the lower left side of the ribcage for more than a month. It wakes me up at night now. It feels like a big bruise, but there is nothing there. I am not an active person at all and there hasn t been any physical strain or pressure on that part of the body. Also started to be nauseous and it s really bad in the morning. I m 60 and was diagnosed with spondylolisthesis 4 years ago. Thanks.