8 month old baby has a rash on her face behind her ears and all over her stomach.

Hard to say: Rashes can be associated with illness or reaction to foods/exposures. You didn't describe the rash in detail or presence of any of fever without which little can be said. You are welcome to start over & provide more detail but many rashes just need to be seen to label.If she spooks you have her seen.

Viral vs allergy: If she recently took a medication or tried a new food or you switched detergent Or babywash, this could be an allergic reaction. Benadryl would help. If she has a fever or respiratory or stomach symptoms, the rash is likely part of her viral syndrome.