Had sex on BC pill in first week while taking a z-pack now experiencing tightness/cramping in lower abs for days, breast tenderness. Pregnant %?

BC pill should work. Generally, a Zithromax Z-pak does not change the effectiveness of birth control pills. One can do a home urine pregnancy test, if worried. A test, done correctly, gives a reliable result 3 weeks after sex (3 weeks after when conception might have occurred), or 1 week after missing a period. If done sooner, a positive result is likely real, but a negative result may be due to testing too early.