Why hasn't she had another period?

my little sister is 14 ears old and three months ago she had her period for a week she has not come back on since is this normal and when will she come back on cus I looks like shes pregant . but she has sware on her life she hasnt had sex .

Don't worry too much. I am 24 now and I was much the same. I had my first (full on) period at 13 and then not another until 16. It depends on a lot of things like genes, stress, diet, and exercise. Everything will b fine swetie. If you have any concerns don't b afraid to check it out with your Dr. There is always a small chance something is happening. But do your research first. Drs are more helpful when you know what you want and what to ask about/for.
Hope it helps.

her period is just irregular because of her age but it will come on monthly as she gets older. she might get hers once every 3-4 months but thats normal for a girl her age.