I picked up my one year old little neice and I felt a weird pain

You need to tell your mother as soon as possible. You're only 13, you need to get proper prenatal care immediately. Since you're so young and aren't done growing there are a lot of risks to both you and your baby during pregnancy. Not seeing a doctor puts your baby in danger.

As Hopinfor4 said, you probably aren't having a miscarriag just from picking up your niece. It's likely you just strained a muscle. I get stabbing pains and cramps all the time when I lift my kids or bend the wrong way. (Of course, I'm almost 33 weeks and I have an ovarian cyst, so pains are to be expected for me.)

Don't worry, but do get to a doctor ASAP.