Period Delayed?

So I was suppose to start 3 days ago and I haven’t. Today I had really light cramps and like old blood discharge but only s little bit. My cervix is pretty low and I’m on birth control and have been for 4 years. The last time I had sex was on Nov. 17 and he pulled out and my last period was on Nov. 16. Obviously I know it’s so super unlikely that I’m pregnant and I don’t feel different and haven’t had an early signs. Just concerned as to what might be going on. Any thoughts?