Electrical Neve Stimulation side effects?

Hi, I am undergoing physical therapy for cervical radiculopathy (caused by overworking the upper back muscles at the gym). One of the modalities used is electrical nerve stimulation. I find that my entire upper back and sometimes stretching into middle back becomes stiff and kind of tight after my therapy sessions. I am suspecting the electrical nerve stimulation to be the cause. Is this even possible? The reason I suspect this is because of the more mundane nature of the other modalities. I cannot think of anything else I am doing that can cause this. Please advice. Thanks,