Pain left side abdomen what could be a reason for that?

First thought is stool retention/constipation. With no more information than that posted it is not possible to provide much of an answer. You are welcome to start over with a description of what character of pain (dull/sharp/pulsing) how long its been there, past attempts to treat, aggravating or palliative factors, etc. With more information a more specific answer could be formed. Or see your doc