My baby has a rash under her armpit. she is two years old. it started on her upper back then disappeared and went under her arm. what is it?

See below: Without a description of the rash itself and any associated symptoms, it is nearly impossible to give an answer here. Furthermore, even with descriptions, rashes often need to be seen and evaluated to make a coherent diagnosis. Talk to your doc.

See a doctor: I would recommend seeing a doctor since I am not there to see the condition.