Period late?

I’m on the nexplanon so I assume this is normal but my period was supposed to start today but it has not started? I’m cramping like it’s coming but it’s not coming? Yes I’m sexually active and yes my boyfriend finishes in me but I just don’t have my period? My back hurts alittle but that’s because I have gone to the chiropractor in a minute and I carry 3 college text books in my back pack. Is this normal?