Feeling terrible flu like symptoms, what could it be?

I’ve had a really bad flu for the past few days. I’ve done an at home Covid test twice and they both came back negative. But I’m feeling really terrible and I cannot fathom the fact that this is just a flu. Does anyone have any idea what it could be? Is it just the flu or could it be side effects from being on Ocrevus? Also, it’s worth mentioning that one of my legs has been extremely stiff and weak ever since I’ve gotten these symptoms. Any help would be appreciated!