I am not a doctor but I have been experiencing the same type of pain and symptoms for the past 4 weeks. I see a physical therapist three times a week. I get severe muscle cramps in my back. These cramps get so tight they cause a chain reaction and soon my cramps include multiple muscle groups. They often cause my pelvis to go out of alignment which then cause Sciatica pain which then causes swelling and the knot that I believe you’re speaking of. I would recommend checking with your healthcare provider for a proper diagnosis. For me, my therapist does some stretches then Ice pack, electrical stem and massage helps. I found a really good stretch that works for me when the pain gets really bad. Good luck and hope you get some answers.