Off yaz (drospirenone and ethinyl estradiol) bc 4 weeks. had protected sex week after "pill period". missed period (due today). tender breasts & lower back. pregnant/withdrawal from pill?

Could be either: give it couple of days if no period check a pregnancy test but it may be post pill changes if concerned see your physician

Likely due to d/c BC: It is not uncommon to have a change in your menstrual cycles - early / late / miss / shorter / longer after stopping oral contraceptives. Your other symptoms can be related as well. However, depending on what u used for protection and how reliably, it would be reasonable to check a urine pregnancy test.