What Causes Irregular Periods?

hi i'm elizabeth.I'm married .2 and half yrs.I had an air travel from india to UK during march.From then, my periods is somewhat irregular.It came on march 28and then may27.I had to again travel by air to and fro for my education.now again its missing.I had a blood test.They told that i had to go thru another fasting blood glucose test.I had an ultrasound.normal.I have been trying to conceive.What is happening to me?