Hi. Thanks for your query and proper history. To think of the commonest causes first, the cause of the pain in the stomach near your belly button can be intestinal infection. As you have a history of small cell lung cancer 3 years ago, the secondaries from this cancer can be present in the lymph nodes of the mesentery and can cause similar pain. This can be confirmed by ultrasonography, tests of the blood, urine and stool and a physical examination by a Surgeon , the treatment will depend upon the findings. Till then complete a 5-day course of an antibiotic with metronidazole, probiotic and activated charcoal with anti-spasmodic tablets wherever necessary.