I have a solid thing in my left side of belly ...I...

Differntial diagnosis of solid abdominal masses on the left side
Abdominal masses could originate from the abdominal wall and the inner organs in the abdominal cavity, among the abdominal wall causes are the lipoma which is a collection of fatty tissue within a capsule in the abdominal wall that are benign in orgin and could be treated surgically, other abdominal wall solid masses are the myosarcoma , Desmoid tumors and liposarcoma. The inner organs could develop solid masses like kidney tumors and  colonic masses in addition to mesentric cyst. Set of investigations are required to diagnose the problem and for assurance and a consultation of Surgeon.
Required investigations
After proper physical exam to define the extent and size of the mass, CBC should be done to detect anemia that is associated with colonic tumors , Abdomin Ultrasound is needed to define the character of the lesion if possible, CT abdomen and pelvis with oral and IV contrast to define the actual size and origin of the mass. MRI might be needed in addition to colonoscope
CAUSES

• Abdominal wall lipoma, liposarcoma, desmoid tumor
• Colonic tumors
• Kidney masses and cysts

Our advice
You are adviced to have a well balanced diet , avoid smoking and have regular exercise