Fertility and periods?

Ge`a
Have already posted one lol but still very concerned still haven't started my period this month and I'm pretty sure I'm Not pregnant. My periods have been really weird lately. Like last month my cycle went to 48 days and now it's going to 49. I use to be every 30 or 31. So I don't know what's happening. I'm 15 years old btw so idk if this is due to age or stress.