Both are acidic fruits, and over increase the concentration(amount) of acid in your stomach. When your stomach digests a meal, the level of acid is greatly, greatly influential. If there is too little acid(drinking too much water or milk), your stomach acid is diluted and takes longer to digest(then you eat
too much slowly digesting food and you feel sick
). When the stomach acid is concentrated, it digests too quickly and doesn`t absorb the nutrients it needs, thus you basically are pooping all the nutrients your stomach missed(diarrhea). The reason it hurts is because the acid is slowly eating
your stomach wall because it has little else to do(your stomach wall grows back, don`t worry)
I would definitely recommend adding some base(grains and protein) to your diet when eating
these acidic foods. I am addicted to citrus and I combat this as my painful payment. These fruits are delicious, I cannot blame you, and nutritious, so they are well worth the pain.