This must be a really scary and frustrating experience for you! Hopefully the wait will be short and your new doc will be able to spare the time to answer all the questions you have.
I cannot speak to why the issue has not been diagnosed (or, it sounds, investigated in depth) until now. There could be a number of reasons why the docs to date believed it was "nothing to worry about." It sounds like you've been to a series of different physicians over the years... so it's worthy of note that while it's entirely possible for one doc to "drop the ball," it's pretty unlikely that several docs dropped the same ball.
Now... as for lytic vs. blastic lesions...
Bone is tissue. And like all living tissues, it is constantly turning over (old tissue dies/new tissue grows.) This is called "bone remodeling." In bone, there are cells called OSTEOCLASTS that are responsible for breaking down old bone to make room for the new tissue. When osteoclastic activity is high, LYTIC lesions result. Areas of low bone density.
On the flip side, OSTEOBLASTS are responsible for laying down new bone. When osteoblast activity out-paces the breakdown work of osteoclasts, areas of increased bone density result. These are BLASTIC lesions.
In Paget's disease, this remodeling is out of sync... so some areas of bone are overly dense and others are not dense enough. Typically, Paget's occurs in older people. There is a "juvenile" disorder, too, though.
The blood tests would have been for Alkaline phosphatase, which is frequently (but not always) elevated due to the increased bone remodeling. Because the blood alk. phos is not an absolute, diagnosis of Paget's is confirmed with imaging (bone scan, CT and/or MRI.)
There are other things that could cause lytic + blastic lesions, which will need to be ruled out. Typically we start with ruling out the more serious pathologies. In this case, differential diagnosis would include metastasis from cancer in another organ. Breast (as you mentioned) and lung will be the first places to look because these both tend met to the bone and they both cause lytic AND blastic lesions. But other organs will have to be ruled out, too. As will primary bone cancers. But there are non-cancerous possibilities, too.
Your oncologist may recommend a PET scan to start. PET scans highlight all areas in your body where "increased metabolism" is going on. It can be a good "whole body" test to flag cancerous tissues anywhere in the body. (In many countries PET scanning is considered "first line" testing, but in Canada some docs are still not convinced PET scans are a useful diagnostic tool.)
However this turns out, it sounds like you have a good oncologist on the case now. I would urge you to do your best not to panic. Remember to breathe. Schedule some fun, dynamic events with loved ones to help keep your mind off things.
And as difficult as it may be, please try to find a way to fit JOY into your routine. Even small things. It sounds trite. And I know some days "joy" is unrealistic/impossible... some days it will take tremendous effort. But whether you have 70 years ahead of you or whether your time is limited, living joyfully helps make the best of the time you have.
Wishing you the very best!