Have your periods always been irregular?
Other than pregnancy, the next thing that comes to mind is PCOS. I am PURELY guessing though.
Not to alarm you or anything but its possible you have an STI (sexually transmitted infection) or a STD (sexually transmitted disease) if you do, most of them are curable so dont panic and wait to see what your doctor says if thats not it, you might be missing your period for another reason, for examplestress, travel, illness or another reason. Anyways hang in there until u see your doctor cause they'll no. Best wishes...12withbaby