My baby has a rash and it's spreading, does she have an infection?

Possibly. Babies can get rashes for many reasons. Infections can cause rashes, but so can skin irritants, like certain pieces of clothing or detergents. Infants can often get rashes for no easily identifiable reason. Almost all infant rashes go away on their own and do not need treatment. If your infant's rash is getting worse, if they have a fever or if they are acting sick, call your pediatrician.
Not necessarily. If your baby seems well & is not having fever, then your baby may not be sick. Some rashes are due to heat and may fade on their own. Some viral rashes may show up as tiny pink spots that do not otherwise bother your baby. Other rashes that spread may be allergic, especially if spreading fast. Any purple/black rash or rash associated with very high fever or ill child should be seen right away.