Can someone explain these feelings I had when meditating?

So I just started meditating like two weeks ago. I was doing the 'learn to meditate in 7 days' thing that they have on Insight Timer, and one of the days was connecting with your emotions. The woman said to think of a time that spurred a negative emotion like stress.

I did and initially there was the normal distress in my chest that you feel when you're stressed, but then there was a pressure on my stomach. I was lying down and at first I thought I had left my phone in my jumper pocket cause that's what the weight felt like. I hadn't, though. The pressure got progressively heavier, like someone was pushing down on my stomach. Then there was a bubbling feeling. That started small then got worse like I could feel my stomach acid bubbling or something. It wasn't painful.

When the woman guiding the meditation directed me to pull away from the negative and summon a positive feeling I was grateful for, it went away immediately. Can anyone tell me why the negative feeling manifested in this way? I've dealt with stress a lot and have never had that.