What Do Delayed Periods And Back Pain Suggest?

I m 26 years old married I had sex with my husband on 06/03, my last period date s feb 14 now one day s delayed. Normally within 28 days my period date gap.i don t want baby now. Last two days i hv little back pain Now I m asking is this pregnancy symptoms or wat doctor. ... plz explain. And tell me how to avoid this