Period is due today but not here? Pregnancy?

My last period was May 6. Well, my period has been irregular since I took the plan b pill bout a month ago. It came early last month. But, I haven't gotten it yet. Should I be concerned? I haven't gotten any pregnancy signs. Nor PMS. like NONE. usually I get backaches before my period and sometimes sore but nothing. I'm not throwing up or anything. I have been SEVERLY STRESSED last month also, and I'm slowly getting over it but now the stress of this is adding on. So I'm thinking its stress thats delaying it. I am ually active and me and my partner are ALWAYS careful. Nothing gets inside me. Should I be concerned? How many days should I wait to be fully concerned? When can i take a pregnancy test, if I really need to? My last period was early as I told you, originally supposed to come around the 15th or so. I dont remember the exact day. Around the teens. Please help.