My 14 month baby has tiny pimples all over her thigh. What causes rashes like that?

Lots of causes... ...Including allergy to diapers or baby wipes or moisturizing cream, or direct irritation by harsh soap. Could also be a virus, which is sometimes accompanied by a fever, but not always. There are other, more exotic causes; your quickest solution is to see your pediatrician or a dermatologist and get a precise diagnosis and treatment.