Sunday's are a mongrel time to have issues. If you have previously been prescribed pain meds, you should still be able to take them. Tamoxifen doesn't generally muck you up when taken with anaelgesic meds, the problem may be what a causing the pain.
That you have already had similar pain is nasty, but maybe should inform what you do next. If the pain is familiar perhaps do what you have done in the past. If the pain feels different (after you rationally filter out all the BC extras) you may have to think about getting more advice.
I miss panadeine.