Nelnet balance increased unexpectedly

the situation: every quarter I capture my total parent plus loans balance to ensure it's trending in the right direction.

I have 6 loan groups. this past quarter my balance increased by $2k to $133k despite making $11k in payments in the same period, and no new loans added. I always pay significantly more than the minimum and this the first time I've seen the balance increase without new loans being applied.

I've called Nelnet and their explanation is that I have been applying the payments to a single loan causing the interest to build on the other loans. they also said something about advancing the loan payments.

my view on all this is that unless there was $13k of interest in that period, something is wrong on their side. they don't agree and keep referring to the payments applying to single loan.

any advice, including if I'm misunderstanding, is welcome.