Reincarnation

Yesterday, on my way home to school, on the tram I heard these two girls talking, at the tram stop, and I overheard the conversation, where one girl said, "I think I was a dog in my past life."
And I stopped to think.
"Hm, that person believes in reincarnation. There are more people in this world who believe in reincarnation that I expected."
I personally do not believe in reincarnation.
I was raised as a Buddhist though, where they believe that life is like a wheel, it never stops, when you die, you are born again, but in a different form, whether you are a human, or an animal.
This is the sad thing, they believe sinners are to be reincarnated into an animal, or something bad/worse in the next life, to punish what they did in the previous life.
I remember being told this when I was young. I probably did something rude, like talk back to older people, and it was believed that if you sinned using your mouth (rude, hurt people through your words, etc,), you were to be reincarnated into a fish, as their mouths keep on moving, and never shuts. And other animals, I cannot remember at this instance, but it used to scare me.
I too am scared, writing this at this very moment. Saying how I do not believe, but I am scared. What if it is true. I certainly hope not though, because then what are we living for?
It worries me that what I do now, will affect my "next life".
It is also believed that people who have diseases, or are unfortunate, are poor, etc. is because of what they did in the previous life. The question is, how is it fair that we should be punished in the next life because of what we did previously. What if we weren't informed about reincarnation. Like, it is coincidental, that I have been informed and educated about the belief of reincarnation, in this life, but then if I didn't, and I sinned, then I will receive my punishments and suffer in the next life, where our memories are erased, where we don't know why.
This really scares me.
Ugh, this whole thing about the after life scares me.
Maybe it's my lack of sleep that makes me think of strange things I do not even want to think about.