Whenever a student comes to me with what might be a medical problem, I always recommend that they seek the advice of their doctor. Yoga teachers cannot assume the responsibility of a professional health care practitioner. Your student may have a problem needing care that you are unable to provide.
There are some safe approaches for you to take, however. You may need to slow her downhave her back off from some of the more intense poses and reduce the number of times that you allow her to jump back in. Many Ashtanga students tend to be the types who overwork and push too hard, and that may be the case with this student. Give her restorative poses, and make sure she is getting plenty of Savasana (Corpse Pose).
You may also need to check her breathing. Often students misunderstand Ujjayi breathing and force the sound of the breath, causing stress. This can strain the body and the mind, leading to further problems. The breath should be barely audible, soft and smooth.
As Ashtanga teachers, we need to remember that the health of our students is more important then the actual sequence. Further, it is important that we help our students find a healing attitude toward themselves and their practice. Do not think that backing off the series or altering the sequence is wrong practice. Instead, look for ways to help your student be safe and free of pain.