What type of rash is this?

So baby has a slight cough (dry) and a runny nose (clear mucus). Her highest temp was 100.8 and nurse said it could just be a cold so we have her tylenol for a fever and MIL recommended Benadryl but it says not to give babies Benadryl (do you guys give it to your baby?). Anyway, I noticed this rash today around 2pm and as of 9pm this is what it looks like

Personally it doesn't look like heat rash and it's also on her upper arms. Not on her chest or face. Please help.