Late period

El
I'm starting to really stress. My period always comes in the same day which should be today but I have absolutely no sign of it. I've had sex a few times but have always used a condom, pulled out and I'm on the pill. But I did just change to a stronger pill like a week ago. And I've been extremely stressed lately. 
Do you think my periods just late because I've been stressed and changed my pill. Or could I actually be pregnant?