what kind of 'bug' is this? Help!

[Chulita]

DD (16mths) has been getting huge fevers since Sunday, 40C+ but only at night. She wakes up with a normal 37Cish temp, it climbs a bit at lunch to around 38.5C then in the evening it goes a bit higher, sometimes topping 39.5C. It's at night that it gets really scary, she wakes up at least once crying and each time her temp's been over 40C, we spend over an hour stripping her off, fanning her and resettling her and in the morning it starts all over again.

I've been alternating Ibuprofen and Paracetamol during the day and then in the evening/night when she peaks I give her both at the same time. She's not eating but will drink milk and tiny amounts of water (and she has wet nappies).

Behaviour wise she's a bit more grizzly than normal but otherwise fine. When her temp goes above 39C she just lies in our arms and won't move/cries when we try to move her and grunts, her lips and nail beds go blue. When we get her temp down again she'll potter around doing what toddlers do.

I've taken her to the doctor's twice and they've checked her. Apparently her lymph nodes are slightly up, her tonsils and ears are a bit red but otherwise they can't see anything wrong with her.

She had the same thing 10 days ago and it took 6 days-ish to clear but now we've got it again.

Any ideas what this could be and what we could do about it?

[CantSupinate]

Virus.
I would try to let the fever run as much as possible, I don't know the exact temperature number to call too high, I'm just saying that the fever is there to do a job and I would be slow to give the Calpol/ibuprofen unless I felt it strictly necessary.

Does she cry at all when weeing, or tug at her ears?

Mine have all had something similar recently, btw, though not so long-lasting.

[Chulita]

She doesn't cry when weeing or swat her ears and I don't think she's teething. I've only been giving her painkillers when it goes over 38.5C but maybe I should hold off til it gets a bit higher.

[CantSupinate]

Well don't believe me, see what it says online (NHS) about when to bring temp down.

Hope she gets better soon.