Everything is possible because the human body doesn't work by clock-work precision . By asking such questions the patients only try to put the Doctors on the backfoot.
When your cycle has a variation of 4 days, everything (safe period and fertile window) will have a variation of 4 days. In short no mathematical advice can be given.
For your latest problem, please get your
beta HCG level done and consult a Gynaecologist with the report.