You need to start of with around 2 grams per day. If after 4-5 days it doesn't get better, up it by 1 gram for another 4-5 days. Continue this until you get up around the 5g dose. Within that time frame you will find the dosage your body needs.
When the symptoms disappear or start to get better you can back down on the grams per day and then settle in on a mantanence dosage.
But what happens is your body becomes very anabolic. The stomach lining holds a ton of glutamine so, as your demand for glutamine goes up because of being in a hyper anabolic state the body starts sacrificing the glutamine. The first place it sacrifices is in the stomach. This is why tren is notorious for heart burn and the like.