Hello Kim Seo Ra
Thank you for writing!
I know it's hard to be 15 and have questions like this that are hard to ask. You cannot be pregnant and your issue is very common.
Changes in lifestyle, stress or just a simple change in body chemistry can change the menstrual cycle. Most girls at age 15 often skip periods or are irregular. Your body is still growing and your hormones are still changing all the time.
If you don't get your period in the next month, it's a good idea to talk to someone in case you may have something going on with your hormone levels (again, this is common). Just explain to them what you explained to us. Nobody is going to think you are pregnant.