So Confused About Fertile Week

Up until yesterday one fertility calendar said my fertile week would start today and another calendar said it would start Thursday  so I started tracking my ovulation yesterday with Clearblue digital OPK. I had a low reading yesterday and a peak this morning.  I recorded my test result this morning in the calendars and my fertility week changed in both calendars to a start date of last Thursday. I'm just so confused and worried I'm never going to get the timing right.