It could be any of the things the moms have already mentioned. It does sound more like a fever blister which is what was mentioned as the herpes virus. These and the other conditions come out when the immune system is at it's lowest point.
Several things can be done. First is make sure she is getting the proper nutrition. Avoid synthetic chemicals in foods such as processed foods, pesticides, preservatives, hormones, and antibiotics. Fast food is the worst because they usually combine the chemicals in the processing with the simple sugars that are bad in white breads and white potatoes.
An absorbable multivitamin/mineral complex would be good for her and you as well. If it is a fever blister, L-Lysine is an amino acid that is supposed to be in your body but is depleted with stress and the immune system drops. It can be purchased at the health store.
Fresh air and exercise helps as well. I detoxed my home and removed all synthetic chemicals about 5 years ago and we don't get sick anymore. We are now able to keep our immune systems strong. All of this is easy and very inexpensive. Let me know if you have any questions and I'll be glad to walk you through it.