Ovulation Calenders

Okay I have three ovulation calenders I have Glow, My Calender and Ovia. All three tell me different s

dates on when to start my period. With Glow I am to start today which I have been feeling really gassy upset tummy and a lot of nausea with a slight headache and tons of creamy thick cm. With my calender it says in two days and so does my Ovia calender. I have entered the data on all three the same I just dont understand why its giving me different days. I dont know which one to go off of? Feeling confused would it be possible to have a test tweeked if I took one depending on what it shows?