Milk allergy?

Our son is 8 months old. With the stress of working full time and raising 4 kids, house work, after school activities ext...my milk production significantly dropped. We started supplementing about 7 months and is now 100% on formula. All of a sudden he is miserable, cries all the time after each feeding, constipated, and now has a red rash on his face. I’ll be making an appointment tomorrow for him, but does anyone else have a Child allergic to milk or have a lactose sensitivity? Does this look like a reaction?