late period?

anna • 14 years old, just a normal life teenager.
the last time I had sex was a month ago, then I had my period! and I haven't had sex since then, but this month, I'm supposed to be having my period and nothing! it's been two days and there's no period.. should I worry? what do I do? please help me ?
​could it still be irregular? I just started like last year, and my sister goes through the irregular cycle and she started before me? I just Really need help..could I be pregnant?! :( 
​**& also, is there a way I can tell glow I haven't gotten my period on that day?