More than likely, your host wants you to enjoy their hospitality and would be devastated were you to suffer an allergic (or other) reaction to something they served.
You really owe it to them, as well as yourself, to explain your situation.
If you know, for example, that you can digest bread made from Spelt or some other grain, you might suggest that alternative. Some bakeries and supermarkets sell kosher baked goods made from spelt, etc.
Perhaps, though you cannot eat bread, you can eat Matzoh. Discuss your dietary options with your physician.
Enjoy your Shabbos.