I usually refer to the askdrsears.com website for rashes. He is pretty good at spelling out what's serious and what's not, as well as giving the proper terminology so you can explain the rash to the doctor's office correctly over the phone! You should always call your pediatrician about a rash that's more than a diaper rash. Most rashes are short-term viral infections that may or may not have a fever with them, but some can be signs of more serious illness so it's best to make sure your doctor knows about it. You might also want to consider HIVES as the culprit. Some kids can develop hives from an allergic exposure, or from an illness. An antihistamine and fever reducer can help with the discomfort. Hives have the habit of coming and going seemingly at will as well, so if the rash disappears one day and comes back a few days later like it had never gone, you're probably dealing with hives. If she were older, and depending on where you lived, I'd also suggest fire ants might be the culprit because those little buggers can swarm up a kids legs and bite in a flash! They usually leave small red bumps that hurt and itch and usually turn to white pustules in a day or two. Most kids will holler when bitten so mom or dad knows to come and help them and it's clear where the red marks came from, but I do recall one friends little boy that stood in the middle of a fire ant hill as a toddler and was bitten repeatedly and seemed to not notice!! Your little one probably isn't running around the yard so fire ants are unlikely!