Back pain fatigue and nausea for a month

[Realitea]

My dm has had what she thought was a water infection four weeks ago. Three courses of antibiotics later (and repeatedly sending samples) she got worse. Turns out the samples didn’t have bacteria so why she was prescribed antibiotics I don’t know.
Back pain has got so bad she’s been in bed feeling awful for all this time.
She saw the out of hours gp who said a ct scan would be a good idea
She then saw the usual doctor the following day who said no but they’ll do blood tests, examination and ultrasound
Nothing showed up
She’s now been sent home still in pain and feeling ill. She’s in her 70’s. I am so worried. What is this? I don’t have much faith in the doctor after them prescribing antibiotics and her not even needing them!

What are her symptoms, just the back pain or is there reason to suggest something else is going on?

It’s still very much like the symptoms of a water infection. There’s fatigue, nausea (off and on) some tenderness in the abdomen but again nothing found.
She did have a temperature in the second week but that’s gone. Strangely for two days at the beginning of this week she seemed better from the last day of her third course of antibiotics but then the back pain came back again. She has no energy at all probably because of the constant pain