Sorry your family is dealing with this @basicallyamy! It is scary when your child first receives a diagnosis.
You are lucky she has been diagnosed as this is a disease that often goes years before a doctor recognizes it. We feel like we are lucky in a way that our son had a major flare with visible swelling as it forced his doctor to recognize that something was wrong and send him to a Children's Hospital. He had been having painful heels for a couple of years which the doctor attributed to over-use in sports. Turned out it was one of the first symptoms of his AS (enthesitis). So when he was first diagnosed, he was having a major flare like your daughter.
Like your daughter, he was also given methotrexate (MTX) as a first try along with an oral NSAID. The MTX had no effect, which is not unusual in kids with AS, but at least in our case, our insurance required that he try it prior to moving on to other meds. In his case, he was finally put on a biologic, TNF-blocker injectable drug (Enbrel). We were worried about this class of drugs and did a lot of research before he started, but in the end came to the conclusion that the inflammation he was experiencing would eventually cause joint damage if it wasn't brought under control.
Enbrel (and the Humira he later switched to and still uses) turned out to be miracle drugs. He went from having difficulty walking to being able to take part fully in the sports he loves. He has been on anti-TNF drugs for 10 years now and has had no side effects to date. He has regular blood tests to monitor any side effects. Of course he has days where he is stiff and achy, but as long as he incorporates a lot of stretching into his morning routine, he is functional.
Each case is different and only your rheumy can tell you if your daughter's case is progressing at a high rate. If you are not already seeing a pediatric rheumy, try to get into a children's hospital for a consult. Children with AS are often treated differently than adults and may even be given stronger doses of meds than adults as their bodies metabolized the meds differently. While many areas don't have pediatric rheumies, you should have access to one if you live near a major city in Calif. Most of the major areas here have at least one.
Best wishes and hopes for a speedy diagnosis!