Aw mama, normal yan sometimes, a few of us noticed mild redness or slight reaction sa baby when we switched or started mixing formula, kaya don't worry too much! But since it's on baby's face and you're not sure kung formula reaction siya, allergic reaction, or something else, mas safe na i-check with your pedia just to be sure, especially since she's so little pa. It could be the formula itself, or even just heat rash or irritation from feeding time, pero better safe talaga with babies! How's baby acting otherwise, comfortable pa rin sya or mas irritable?