I don't know what this is
So about one year ago, I went to the lake with some friends. I kinda swallowed a bit of the nasty green water and soon coughed it back out. It wasn't exactly "throwing up", no food or anything; just the water and syliva. However, this was just the first time. I have done this exact thing (caughing out syliva) quite often. It is related to stress, because it happens everytime anything important to me is about to happen. It's that nervous stomach feeling and walking around makes it worse so I have to sit down. Also, stretching seems to relieve it temporarily. This wouldn't be a problem if I was just born this way; this never happened to me before and its been happening all throughout the year and it is very aggravating. The kind of thing that I would never stress over is multiplied and I feel like I'm about to throw up all day, and eventually end up coughing out spit.
I am 17 years old, a male, and perhaps my weight has something to do with it. I am around 133lbs of average 17 year old height. I do not 'refuse to eat' or 'purposely throw up foods', I just have strong digestive system and am naturally light. Still, I wasn't always this light, I went on the opposite of a diet to gain weight and when I started to lose it again, this started to happen.
What does this sound like?