I’d probably request a CT scan, because I’m not sure my doc would approve a PET scan initially. I do think this is worth pursuing. Even if you find it’s not a big issue. Worrying about it over time is not good for you!
I think a CT scan would rule out a wider variety of causes than a regular X-ray, but I’m not a doctor.
I’d be considering degenerative disk disease, especially because of the letrozole. If it’s that, then an adjustment to medication might help stop further damage.
A CT scan would also help to see if there were any organs pushing on the spine causing the pain. That would prompt further investigation.

I think it is best to be as clear as possible with doctors about symptoms and onset. And things you’ve tried, like you mentioned.
They need “hard” data to get these scans approved by insurance. That could be when the aching started (guesstimate is okay) or perhaps you had mild discomfort prior to a certain date - then you realized it was constant. Does it happen all night, or only after sitting for hours, or now with every activity 24/7 - all this helps them eliminate what might be causing it. Along with telling them your physical symptoms, also include how it’s affecting your life. More anxiety, avoidance of certain activities, limiting your physical activity - Doctors (should) know that if it’s affecting your daily life that it can lead to even more physical problems as a result.
Wishing you the best!