New to this app

Were ttc and this is now our fifth month. I took a clear blue digital and it said not pregnant although now my period is completely missed. I'm 6 days late. It should have come and gone by now. Any ideas why my period could be late? I haven't been stressing or anything. Last week at 13 dpo I got a bfn and so I assumed there was no way I was pregnant so I stopped stressing about it and now my period is so late. Any ideas why or what I can do to make it come?!