1 wet diaper per day

Good evening!

We have three kids aged 6, 3, and 11 months.
The youngest one got a fever from sixth disease over the weekend of August 22nd. After that, the fever went away.

Since that Wednesday, she has been drinking and urinating very little, averaging 220 ml of fluid per day, and only one full diaper. So, it's been like this for over a week now.
She still has the same diaper on now (19:00) as she did early this morning, and there are just a few drops in it (a small wee).
I went to the child health clinic this afternoon, where they also noticed that her growth and weight are going down in the curve.
They advised me to call the GP. I went there late in the afternoon, and they concluded that there's nothing wrong at the moment and she's not dehydrated.
However, they said we should come back right after our holiday, between September 3rd and 15th, if it's still like this, and we'll be referred on.

I'm not quite sure what to make of this. I'm not usually worried, but she used to drink really well, and now she's urinating really little... what do you all think?