Yes. I'm 13. It wasn't my fault, I didn't expect it to happen, but he pressured me and I said yes. My mom found out about it and told me it was practically impossible because I had just got off my period, but my health teacher said that anyone who thinks that is sadly mistaken. :/ My sister in law said that if he kept going (especially if it was too big) that it could have leaked out the sides. I've been having a lot of symptoms like being tired all the time, peeing every half hour, moodswing, And when I go to the bathroom, There's white stuff in my underwear. It's not a lot though, just like when you get really scared and you pee yourself. It's not itchy or anything though. I honestly think this is all happening because I'm majorly paranoid. I'm really scared. I asked my mom to buy me a pregnancy test for my friend and she agreed. She was so mad when she found out that she said she would never discuss it again, and if I asked her to buy it for me, she would say no without hesitation. All of my friends are supporting me in this, and they know how stressed I've been. I think that if I'm not pregnant that my body has some serious issues. Please help!

Your teacher is correct. The odds increase toward the middle of your cycle, but you can even get pregnant during your period.
The white stuff can be anything - many women secrete some mid-cycle, but it can also be a yeast infection or even a STD - sexually transmitted disease. Yeast is more like cottage cheese. Mid-cycle secretions are usually more clear and stringy and gluey. STDs will vary a lot.
So now you know what you got into. It might be too soon for a test, you might be worried and that can have an effect, but you really should be tested for disease and BE CELIBATE until older!