Sometimes mild, often severe. People living with AIP have said the following (although your symptoms may be different):
It feels like someone is slicing your insides with a straight razor, while pouring hot lava through them. There is really no way to explain it- R.C.
Being kicked with a steel-toed boot- K.E.
It feels like I am digesting glass shards- J.N.
A 600-pound person sitting on my stomach and twisting it like a wet towel to remove all the water- B.N.
Demons chewing their way out of my gut and standing in a pile of fire ants- R.K.