Severe back pain during infusion

I have been taking Vyvgart for a year now and it has been working really well. However, during my last infusion, i suffered a severe side effect being massive back pain, which started 5 minutes into the infusion, got progressively worst then after the nurse stopped the infusion, after 5-7 minutes of intense pain, it slowly subsided, after another 5 minutes the pain was gone. They nurse/pharmacy wondered if it was a bad lot of the medication, as I had never had a bad reaction previously. Has anyone else suffered from such a reaction, and did it continue with future infusions? This week we are using a different lot so i am hoping to go back to no side effects.