Concerned about gadolinium reaction and deposition disease from mri. one time exposure 5 weeks ago. no immediate reaction. now 5 weeks in, possibly symptoms of new knee pain, feeling of being hot but no fever. any way to tell if its from gadolinium?

Not likely: Gadolinium deposition disease is not reliably proven to occur (https://www.jacr.org/article/S1546-1440(19)31295-5/fulltext) . It would be better to assume that something else is going on with your knee and follow-up with your doc about it. If it is hot, you may have a joint infection, which can be very serious! Please get seen today at an Urgent Care or ER if your doc cannot see you.