To be honest it sounds like sacroiliac joint pain. Have a read of this.
http://physioworks.com.au/injuries-conditions-1/sacroiliac-joint-painHave a chat with your Dr. Perhaps a referral to an exercise physiologist might assist with exercises to decrease the issues especially after running.
It might well have nothing to do with tamoxifen. I have had this and also recently discovered I have some bulging discs and spurs in this area. All there pre treatment but of course we all worry...so I had a scan in the end to just settle that fear. It became worse after as I wasn't doing as much exercise especially stretches...something I have rectified slowly. Kath x