late period?

K
Haven't had sex so why is my period late ? There is no chance of being pregnant. I have had some cramping but nothing too bad...  I am usually never late & <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">eve</a> is pretty good at predicting when my period is to come. 
Factor: I have been getting test done such as x-rays or MRI done, does that mess with my cycle ?