Anxiety, if that is your problem, isn't rational. That's why it's a mental disorder, not just a bad day. But we can't now if it's anxiety or something else. Rather than go to the ER, do you have a doctor? Have you seen a cardiologist? Either, or both, can give you a complete evaluation and see if there's anything going on. Here's the question I'd ask -- did you feel anxious before this heart thing started? Did you have phobias? Bouts of anxiety? Insecurity? That would indicate an anxiety problem brewing and now it's broken out. But if you didn't feel any anxiety before this, then maybe there is something going on and it just hasn't been found yet. There are so many things that cause symptoms that mimic anxiety attacks, from MSG in Chinese food to thyroid problems to blood sugar problems to food allergies to hidden viruses to any number of things. It takes a dedicated doctor to eliminate all these before diagnosing it as anxiety. But assuming it is anxiety, then going into therapy with someone who specializes in anxiety treatment is your first step before this become chronic. And it will help if you're exercising regularly and you might look into meditation, which can even out some of what you're experiencing.
Even if sometimes it just happens. Like today i came home from work. Very tired. And now that im relaxing is when i started feeling everything. Can it also be stress on my body?
You are living in fear, because you distrust the heart tests that said there is nothing wrong. The fear makes you over-analyze your body, which turns into a vicious circle when you notice something that you think is wrong Since the tests said there is nothing wrong, .I am guessing that the only time your heart might beat fast is when you worry that it doesn't beat correctly - or maybe it doesn't beat fast at all and just seems that way.
Obviously your lungs know how to breathe properly even if that seems to not be true, so try to find a way to focus on the fact that they even work when you are sleeping all by themselves.
If you can't find a way to relax on your own, then you might want to try a few sessions of therapy.