its is not haemmeroids or a fissure, there is no bleeding , and it is not just in the anus. I feel a fullness in my stomach due to gas and when the gas exits it is quite warm.
Short of there being any food tolerance issues - you should probably follow food guidelines as laid out by the USDA in the USA or the Canada Food Guide (in Canada!) to ensure you eat a relatively well rounded and healthy diet.
http:// fnic.nal.usda.gov/nal_display/index.php?info_center=4&tax_level=2&tax_subject=256&topic_id=1332
As for the burning sensation, there are a myriad of causes -
Hemorrhoids - A swollen vein or group of veins in the region of the anus
Fissure - A long narrow opening in the form of a crack or groove, e.g., any of the spaces separating convolutions of the brain
Anal fissure - A natural crack or tear in the skin of the anal canal. anal fissures may be noticed by bright red anal bleeding on the toilet paper, sometimes in the toilet. if acute they may cause severe periodic pain after defecation
Irritable bowel syndrome - A widespread condition involving recurrent abdominal pain and diarrhea or constipation, often associated with stress, depression, anxiety, or previous intestinal infection.
Crohns disease - A chronic inflammatory disease of the intestines