Help please

I either don't know how to use this app or I've just got dates all wrong. My period hasn't arrived, this app says it's suppose to arrive around the 28th of April, i don't rely on this app a lot. However I did notice that my date wasn't even a full four weeks after my last period, it was only 3. I don't know if this is making sense, but this app is making me panic. I don't know why I'm panicking as the last time I had sex with my boyfriend we used protection (condom) we always check it before we use them and they weren't broken. So I'm panicking because my period is not here yet!