Hi, I know how you feel, I've had these all my life... when I was a teenager I never paid them much mind, but when I got to be your age, I had a long bad spell of them that freaked me out, I went to the doctors and they told me my heart was fine, same as you've been through.
If you read the "heart palpitations 101" series up at the top of this forum page, it has a whole lot of useful information about what you're going through and what it is. Every symptom you described is definitely common with benign palpitations, and the fact that you can trigger them with certain body positions is actually fine evidence that it really is nothing serious.. lots of us have experienced that. I for instance get them more often when I'm bending over or lying on my left side.
Stress is definitely a factor, and it's not so much a chicken or egg argument as it is a vicious circle. Anxiety definitely aggravates the palps, and if the palps aggravate the anxiety, it goes on forever. It's not the only factor, gastro-intestinal disturbances (like my IBS, and probably your UC too) can also cause them... but from what I read of your experience I think anything you do to reduce your stress will reduce your palpitations as well. For me, it was better diet, exercise, and tackling my fears head on, but different people have different things that work for them.
I was surprised to learn that everybody has palpitations, as many as 100 a day... but most people never notice them. We tend to think our hearts are like clockwork perfection, and that we have to depend on that, but they are not. But we can depend on them anyway because our bodies have redundancy features that automatically keep minor hiccups from becoming major problems... we are wonderfully and terribly made.
If the doctors are telling you your heart is healthy, then it is. The real problem is likely the anxiety. I know it was for me... when I got worked up about my palpitations I was having spells where every other beat was off that lasted for minutes, all day every day, thousands of palpitations per day. After tackling my anxiety issues, they just fell away to almost nothing. Anxiety sometimes rears its ugly head in my life still, and when it does, the pains and palpitations will sometimes come with it, but now that I know that that's all it is, it's easy to dismiss and it never gets as bad as it was. I hope soon it will be the same for you!
God bless!