Diffuse pain on left side. Pain on deep palpation on lower side. What could be it?

The symptoms you've described, diffuse pain on the left side of the body and pain on deep palpation on the lower side, could be caused by a number of different conditions. Some possibilities include:
Musculoskeletal issues: such as a muscle strain, tear, or sprain; or a problem with the bones, joints, or tendons in the area.
Internal organ problems: such as a problem with the kidneys, spleen, or colon, which are located on the lower left side of the abdomen.
Gastrointestinal issues: such as diverticulitis, inflammatory bowel disease, or a stomach ulcer.
Nerve problems: such as sciatica, which is a condition that causes pain, numbness, and weakness in the lower back, hips, legs, and feet.
It is important to note that this is not a definitive list and there might be other causes for the symptoms you described. It is important to see a doctor for a proper diagnosis and treatment. They will perform a physical examination and may order tests such as imaging (CT, MRI) or blood test to help identify the cause of your symptoms.
Well, the good news is that both the appendix (the "fix it or die now" problem) and the gall bladder (the "experience pains that can rival cluster headaches" problem) are on the right side, so it's probably not either of those, which means THIS IS SOMETHING TO TALK TO YOU DOCTOR ABOUT. We're just helpful internet weirdos.
Ask a doctor not the internet, I went to hospital on New Year's Eve with what I thought was my bulging disc flaring up but after an MRI scan it turns out the back pain is caused by a tumour in my spinal canal.
Maybe something's up with your spleen? It could be inflamed?
Opinion
3Opinion
Kidney? If there's no external bruising, see a doctor.
Find and ask a doctor immediately
It could be a doctor's visit
Superb Opinion