weird cycles??

Brit
Hi I need help, I can't afford to go to a gyno, I'm 18 and have no parents etc. in June I missed my period by six days, wasn't pregnant and the got it. Since then to now if gotten my period every 18 days, when it used to be like 28 days per cycle. I was supposed to get it three days ago but I haven't, and don't know what all these changes are? HELPP