It actually is a good sign that you responded so well to the first anti-TNF. It means you're more likely to respond well to the next one. And you should consider yourself VERY lucky that it worked that well for you.
Don't lose hope!! There are several more biologics out there and more in trials. And you don't know if you're flaring or if your pain is muscular or from something else. Just because you have AS or axial SpA it doesn't mean ALL back pain is from that. For example, I have a lot of muscle spasms and back pain unless I do regular PT and exercise.