I feel pain when defecating

Dear doctor; When I go to the toilet, I get a burning sensation during and for a short time afterwards in my anus. It is very painful and I am beginning to get it quite regularly. I do not eat much spicy food and I am not a heavy drinker, although I may have a couple of pints of Guinness a few nights a week after work. Is this about diet or there is some underlying health problem?
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Times Reporter