pregnancy scare..

so me and my boyfriend had sex on Christmas (I know kinda tacky) which was about 2 weeks ago. I have 2 <a href="">period tracker</a> apps.. one tells me period is late by 3 days and the other says I should start today.. and I haven't. I'm 16 and my mom doesn't know and I'm really freaking out.. can someone give me advice or tell me if I'm worrying over nothing? I'm really scared.