I suggest you switch back to
the first brand before doing anything else. Yes it may be a reaction to the active ingredient but it may also be something in the binders (the other ingredients made to bind and shape the pill) that you are reacting to. Some of the additives may cause an allergic action, that you may be unaware of. It's not uncommon for people to get some unpleasant reactions with different brands. It may be worth switching mid package, just to check before seeing your oncologist, which is the next step. Best wishes.