Thanks ladies,
I think I will give it a few days and then go get her checked. She did have unexplained fever last week for a whole seven days, but that has gone away on its own as the doctor said it would (apparently she had seen a few patients with similar symptoms last week).
The first night she complained about it, she seemed genuinely in pain as she was up to 2-4am trying really hard to go back to sleep, but every time she drifted off she'd abruptly wake up with more pain. She finally settled when I gave her some meds.
The second night, she was awake at around 3am complaining on and off for an hour and I was able to massage her back to sleep. But that night, I couldn't figure out if it was genuine pain or just a little discomfort or nothing at all!.
Last night she woke in the middle of the night and complained about leg pain, but was able to go back off to sleep quite quickly without needing any medicine or massage. I'm not entirely sure if she was just saying it for effect last night as she does like to re-enact past experiences at the moment!
The leukemia thing sounds awful - the things worst nightmares are made of. So sorry for your friend. I will definitely take her in to get her checked if she carries on complaining about it much longer.