I don't want to be alarmist, so please don't worry, but if you know something isn't right and you aren't getting anywhere with GP, take her to A&E. I had to do this with my DD last year. I knew she was ill, things were not getting better, and the GP concluded that it was all manner of things from excess stomach acid, pulled muscles, etc.
She was tired - falling asleep all the time - really pale, hardly eating. She went on holiday with her Dad (I thought she was okay to go at that point, it was early days) and ended up in hospital abroad having tests. She came home, I took her back to GP who continued with broad speculation. In the end, out of despair, I took her to A&E myself as I was worried and not getting anywhere. I had to do this several times. Bloods showed nothing, but she was definitely ill. This whole debacle went on from October-January. They did refer her from A&E into a children's assessment clinic where they began to suspect she had Crohn's. We were still being sent home whilst tests were scheduled.
However, in January things took a worrying turn when her eye drooped down her face. I thought she'd had a stroke. My Dh took her into hospital (I had hideous flu bug) and they kept her in Children's Medicine for a few days. she saw all manner of specialists. They still thought she had Crohn's. Things like her 'pulled muscle' according to my GP showed up as severe swelling on her joints when she was given a number of scans. They were doing lots of bloods. We finally got sent home to wait for more tests, when I was called by the consultant. Something unexpected had shown up in the bloods. We had to go straight in and we were sent onto the Children's Oncology where we were told she had leukaemia.
I was also told that it is so rare that GPs often miss it because they can go through their whole career without encountering it.
I had to push and take her to hospital myself because I knew something was wrong. If you are certain that something is wrong, then do the same.
The hospital was fine - I told them I was worried and my GP hadn't helped. They tested her thoroughly from the first time I took her in. They weren't annoyed that I'd taken that action.
I really didn't want to post as I know how scary this must sound to you, but I thought in the circumstances that this story might help you. Do not be afraid to take matters into your own hands if you feel the GP hasn't resolved things for you.
I hope everything is okay and I am sorry if this upsets you. It could be all manner of things (the symptoms are indicative of many illnesses, aside from cancers) but ultimately, we know our own children, and you seem to be clear that she isn't well.
Best wishes.