More than likely the Tamoxifen. I have had chronic lower back and left hip pain since 4-6 weeks of starting that drug over 16 months ago. It never goes away. Not bad during the day while I am moving around but as soon as I stop and sit ouch! I also have a lot of muscle burning like pain around that area if I have been still for too long.
I took krill oil, magnesium and vit d for 10 months and it didn't make any difference but it may for you and it's worth a try.
Nurofen will give the pain a nudge but doesn't last long and nobody wants to take too much of that.
I have regular breaks off Tamoxifen but only stay off for a couple of weeks which is enough to start easing the pain but not long enough for it to go altogether.
Hope it gets better for you soon.