Like I said, you're only 14 so its most likely just down to your age and puberty which cause irregular periods. Could also be rue to an STD so I would go get checked for that. Other likely causes are cysts etc too much not as likely at your age.
It;s not a miscarriage. It's far too early to even show up on a pregnancy test if you're pregnant never mind it to miscarry. The egg would literally just be implanting now if you conceived. I wouldn't recommend hitting yourself on the stomach to bring on a miscarriage either - that's just stupid.
You should be wearing a condom or abstain from sex altogether if you don't want a baby. You're only 14 so you're still a child yourself. Most likely just a period that's came early. It;s normal at your age to have strange and irregular cycles because you're body is still growing and going through puberty.
A couple of more weeks. And I already planned not to have sex for awhile.
When are you due to get your period? If you just had sex on the 17th, you would only have just implanted in the past few days. Its extremely unlikely your body would even know it was pregnant, let alone miscarry. I know you said you've heard it all, but if you don't want to have a baby, you have to make the effort to either get on birth control or not have sex until you are ready to raise a baby.