Oh, @Mellyb, it's a potential for all of us who've had early stage disease. I'm amongst the luckier group at this stage, but with a definite lack of trust in my body.
I read a lot of posts on an international forum called Inspire. Not sure if you've heard of it. They do have a metastatic group. A lot of what they talk about re tumour markers is not to take tgem in isolation as they can sometimes be influenced by other inflammatory processes. So, (a) another reading in a couple of months, & (b) in conjunction with scans. Most tumour markers, I believe, aren't overly reliable in breast cancer.
Thinking of you x