It is rare to have a reaction to a stoma bag as the materials that are used are constantly tested and evaluated for skin friendliness. If you have suffered from a skin reaction the first thing you should do is check that you have not used something different in your normal routine e.g. changed your usual soap or barrier wipe. If there is no difference in your normal procedure, you may want to try a different product to see if it suits your skin better.
A no sting barrier film like LBF forms a protective film that shields the skin from damage and provides an ideal base for the adhesion of your stoma bag. You might want to try this and see if it eases the problem.