I hope you are still around on this website, if not posting, at least reading. Your post is from back in July of this year, but I hope you still get to read this.
You are young, only 20, so I almost feel as if I am giving advice to my own son. Yellowish greenish mucus doesn't sound good to me and it doesn't sound like anxiety or bad diet either.
I don't think what you were having was a heart attack, but you could have a potentially serious lung or heart condition and you should pursue further testing and treatment.
Ryan, I have recently been through one of the toughest fights of my life trying to get my heart condition (PFO) diagnosed and treated. My advice to you is to continue to be persistent. Your health and your life depend on it. Maybe it is nothing, but maybe it is something serious. If nothing else, have all the testing done so you can have piece of mind. There are some types of holes in the heart, like Atrial Septal Defects (ASD), that if big enough can eventually cause serious lung complications.
You COULD have something serious, Ryan, and as a mother and as a patient I encourage you to pursue testing with pulmonologists, cardiologists and anyone else necessary. Have the cardiologist do an "Echocardiogram with Bubble Study." Simple echos don't usually detect holes in the heart, but when they inject the saline bubbles, the hole(s) are more easily detected. I hope you have good insurance, but if not, it doesn't matter. Your health and your life deserve the best treatment that money can buy. You can DEMAND tests from doctors when they are trying to brush you off because they think you are too young or too seemingly healthy to have a heart condition.
Fight the good fight for your life, Ryan. It may be nothing but it may be something serious. Persistence and perseverance are qualities and skills that you will have to use many more times later in life anyway. This is a perfect situation for you to put these skills to practice.
I wish you all the best. My hunch is that your situation is serious (although my sincerest hope is that it is not). I truly hope that you will keep us posted, but, if not, I wish you all the best in your quest and your future.