The burning sensation is from the anus. It becomes more intense after a few hours of sitting. It sometimes smells of 'sick/vomit' (much less often) but mostly faecal smell.
I am really unsure what it could be - when checking symptoms of anal fissure, it says bleeding, itching and malodorous discharge. There is no bleeding present. I do not know if there is any discharge present as when I wipe to check there doesn't seem to be anything and underwear is clean.