Referred pain around the bottom *could* be linked to a ruptured lumbar disc if the leaked material is pressing on nerves that affect that bodily function.
You mention you have an MRI in 10 days. I think - if it were me with my past experience - I'd be tempted to play up the 'altered sensation' and get yourself to casualty tomorrow with a view to getting the MRI then and a much quicker diagnosis.
Remember what I said earlier about if an op is required, a delay can make it worse than useless. This is because the nerve damage continues.
Really not trying to scare you, I thought long and hard about posting this reply - as I said, this is what I'd do if it were me.
Good luck.
Noodle. x
Edited to add - cross posted and I see you already know what I am alluding to.