I offer my opinion for You to explain rectum burning.
It appears, that You used too much citrus juice with olive oil.
OR Your bile production is too weak, failing to neutralize oo/juice+ stomach acid mixture.
In either case the liquid entering to Your small intestine and later Your colon is too acid causing burning sensation at rectum.
Next time try little less juice! How is Your body weight? If You are smaller than average You could reduce all quantities!
It is funny, that everybody here talks about same quantities of olive oil and juice.
Those quantities are good for average size person. Others should increase or decrease accordingly.