so glad you got some answers today hun
you had no reason to doubt what you doctor had said in the past, so no reason to think so this time
i am sure the dematogist would inform the doctor of your consultation anyway so perhaps discuss it next time you see your GP

if she is going to be on AB's for ages, perhaps so pro-biotics would help as well hun
some in-liven would help restore her flora
also switch her to a natural toothpaste as the commercial brands of toothpaste can actually make perioral dematitis worse!
washing with goats milk based products can also help even more so that QV and other soapless washing products- you can get goat based products at priceline now!!

also extra vitamin B and Magnesium and zinc will help the body repair- you can get chewables
flaxseed oil in her diet can also help and is an awesome source of Omega 3,6 and 9- flaxseed also aids the skin to revitalise ( you buy it in an oil and can drizzle it on greens, and hide it in food- you dont need much per day- just dont cook with it

Interistingly skin inflammation (such as dermatitis) and gastrointestional symptoms (such as diarrohea) can go together and be a sign of a food allergy!!!!
common foods that are culprits include eggs, milk, nuts, soy, fish and WHEAT!!!!
sounding familar??????????
I really think it is worth checking the link between her dermatitis and gut issues in the past!!
A natropath may be a good practioner to seek help from for both of these concerns

so glad you have got some answers to treat the problem, the hard bit now is to try and find out why the problem occurred to start with so you know what to avoid!!
hugs hun!!!