Two things for perspective.
Firstly, you said you were checking your heart rate while resting. Was there any particular reason for this? Or are you, like me, a cardiophobe who impulsively checks pulse? This alone can cause your heart rate to rise significantly. I find this is partially due to the fact that when you check your pulse, you typically stop breathing or interrupt your breathing in some way. This can cause an erratic heart rate.
Secondly, you are on a hypochondriac board so I'll assume you, along with me, and everyone else on here, are a hypochondriac. No perceived stress does not mean that you are not stressed. As an anxiety sufferer for years, I have felt fine and then out of the blue I get a random palpitation or heart flutter, or my heart rate will increase for hours, for no reason at all other than subconscious straining on nerves.
Next, I'd like to put some more stuff into perspective.
Firstly, dizziness is a typical symptom of anxiety. I've felt a considerable attack of dizziness today, for instance, complete with heart flutters and an impending sense of doom.
Secondly, what do you mean by "odd breathing"? Because in a lot of cases, being conscious of your own breathing can wreak havoc on your heart rate, your breathing rhythm, and it can induce many symptoms including dizziness, pins and needles, and shortness of breath.
Also, have you already had a medical workup? This is an important basis for all things symptom-related. Once your Dr. has given you an examination with tests, it is the sign that medically speaking, you are fine. Also, fatigue can be caused by anything really, especially anxiety. A fear that something might happen does not mean something will happen. Your mind is not reality - reality exists outside of your mind, and your brain does not have the ability to alter this. Have you ever passed out before? Or have you ever had a heart attack? If the answer to these questions is no, then your chances of having a scary event occur is absolutely low.
I understand entirely it can be hard to tell yourself these things when you are feeling anxious, but please look into some methods to counter the negative attitudes. My CBT therapist has been a valuable asset for me - if you can't afford that, then there are plenty of CBT workbooks out there, in particular When Panic Attacks
by Dr. David Burns.
Here are some stats for your viewing pleasure to show you the likelihood of a heart problem at 15. http://www.buzzle.com/articles/heart-attack-statistics-by-age.html
Notice that age 15 is not even listed, and the youngest age listed (45) represents a tiny minority of people who have heart attacks. As I've said before though, get a medical workup, testing if necessary, and report things to the Dr. or your school nurse.