She found the stretching session a bit too much
Getting a good stretch after a long and arduous workout session can bring immense relief to your burning muscles. However, no matter how good it feels, maybe you should still keep control of your facial expressions.
After all, someone might misinterpret what is going on if you do not. This lady looked to be having a pleasurable experience of a different sort while getting her shoulders stretched. We imagine it must have felt like quite a relief, but she should still try to keep it together.