HI, Thanks for asking . I understand your concern. YOu have left sided back pain severe enough to disturb you. After going through your information I feel the cause of such a pain in your case seems to be- - A simple sprain of back muscles.. which will resolve with home care suggested in the end. - Pressure symptoms of the spleenic mass you have. IT is pressing on the nerve supplying middle & lower back & causing the pain. - Another logical possibility the considerably big mass stretching the line of gravity in front.. while the back muscles pull it back to maintain equilibrium.The muscles are thus over strained & get painful. - Nerve root compression at spinal level due to inflammation degenaration of vertebrae/ inter vertebral disc can lead to such pain. * You need consultation of a gastroenterologist/ orthopedic surgeon . MRI of the part would pin point the cause .. this will follow specific treatment. Till then you can try.. taking rest/ applying muscle relaxants locally( no massage please )/ pain medicines over the counter/ avoiding bending or weight lifting while you can get appointment of a doctor/ If the pain is unbearable go to ER for preliminary management. Thanks.