Is it normal to have a slight burn whilst defecating? (not talking painful one, just gassy type)? is it more normal with ibs or not normal at all?

Burn: This can happen with IBS. Rule out anal fissure and hemorrhoids

Common: This can occur because of certain foods that are eaten. Also sometimes a yeast infection. Spicy foods should be avoided. Add yogurt to your breakfast every morning for a week. If the problem still persists see your family physician for an exam and maybe some local rectal medication. Anal fissure will cause a lot of pain and/or bleeding. Sounds like you have had a very good GI workup.