A Question About My Eating/Metabolism...?

So, I'm a 15 year old girl. I don't really exercise too often in an organized fashion. I do occasionally ride my bike, and I go up and down stairs at school. Other than that, I don't get too much exercise. I weigh around 110 lbs and I'm 5'5", which is a healthy weight - I'm also a petite person who doesn't generally gain too much weight. But, I've noticed something. I seem to have a sort of chronic constipation - my "excrement" is always hard and most of the times comes out in pebbles. It's normally very dark. Then, there are my eating habits. I eat like A PIG. I'm like a bottomless pit, I eat as much as a lot of my guy friends. To be honest, I seldom eat enough vegetables and fruits. I eat pretty badly, now that I think about it. I seem to drink a healthy amount of water...given all of this information, I want to know why I eat so much, never gain a pound, and have irregular bowels. I usually poop once every two days - occasionally the wait is a little bit longer. But...will someone just provide me with a few reasons why I could remain the same weight constantly and eat so much junk? I can't have a high metabolism since my bowels are irregular. In order to have a high metabolism, you would have to poop a lot too, right? I'm just confused. And no, it most likely can't be pregnancy. I'm always worried of a freak chance but my boyfriend and I have NEVER HAD . We've only done type things. So, unless his has been inside of my body, or if his sperm has been put near me (which it hasn't), I don't really think pregnancy can be considered a reason for all of this. Any suggestions are welcome, please don't be rude. Just in case anyone has anything to say about my boyfriend and I, we are very safe about everything. We are best friends, have been for almost two years. I wouldn't do anything with him if it wasn't a stable relationship. Like I said, any suggestions are welcome; please help me out! Thank you so much.