I do know of a lady who just couldn't take tamoxifen due to migraine issues. I guess it might be too early to tell if its the medication. Ibuprofen might be a good option for pain. Yes go to ED if no relief and contact your oncologist Monday. Hope things improve soon.
The best way to tell if medication is a short break then restart and see if happens but speak with your Dr about doing that. It may be the sudden drop in available estrogen...so like menstrustion changes. ..it might be something tjat eases off when your body adjust. Your body may well still be doing it's cycle thing regardless.