please please help

so I'm on the pill. I am very good at taking it on time every single day. This month I've had just a couple of late nights where I took it about 20-40 minutes late. Also this month I've only had sex about 8 times though too, and the guy never gets close to finishing inside me. 
But every couple of months (this happened 2 months ago too) my period just doesn't come. Like I feel all the symptoms of it, but there's no blood or dark brown discharge (I only really get dark discharge because my period was light as is and then birth control made it lighter). I took a pregnancy test before my period last month to make sure and it was negative. I just don't get why this keeps happening to me, I have no symptoms of pregnancy and I feel totally fine, I'm just confused to why my period hasn't came. Typically it comes (period week starts on Sunday) around Wednesday but it's not coming and I'm slowing stressing out. Anybody have some reassurance for me?