You state that you have been diagnosed with gastritis and duodenitis
I am assuming this was after you had endoscopy and biopsy. Hope
you were also tested for H.Pylori which is very common with the above
conditions. The biopsy would show this and also there is a breath test
to diagnose this. I am also assuming that you were precribed a PPI of
some sort (Proton Pump
Inhibitor) like Omeprazole, Lansoprazole
Getting GERD symptoms after eating Pizza is almost like test for reflux !
Cheese contains calcium and it can stimulate more acid production.
Tomato sauce (Tomato products), caffeine in coffee and tea, alcohol,
citrus drinks and fruits all add to acid stimulation.
I am sure you probably have some inflammation of the lower esophagus
at the junction of the stomach.
Laying flat makes it easy for the acid to travel back up into your
esophagus because of the weak sphincter muscle.
If you do take the PPI, make sure you take it about an hour before you
eat ,twice a day for 4-6 weeks or even longer if needed till you are
I wish you well.