This happened to me too - I know what it is
I'm new to this forum and I have to comment on this post - this happened to me as well except even worse. It is a body detox. For me it happened during a 3 day juice fast and suddenly my heart started pounding and I thought I was dying. My heart was pounding, I was shaking, hyperventalating and felt that my whole body was "tightening up". I didn't want to drink any more juice and was really freaking out. It just felt like a heart attack but I thought, it couldn't be that, I have never had heart problems.
I found out later that this is what's called a gallbladder attack. When your gallbladder/liver detoxes, as during a juice fast, you may have a gallstone that actually moves and blocks the opening, and may cause nausea or some kind of an "attack" like I had. I continued on the detox program and a few days later did a liver/gallbladder bomb and passed many, many gallstones. It was hell for a few days but I lost a ton of weight and ended up feeling great at the end. If you have a toxic liver/gallbladder you can also experience a lot of bad emotions - depression, anxiety, worriedness, sadness, etc. These, amazingly, went away to a great degree after I cleaned out my liver and gallbladder!
I almost went to the ER when I had this "attack" but luckily I had people around who helped me through it. I didn't want to go to a hospital where they would give me drugs and do bad things to me. So first thing I did was take some hawthorn berry extract which helped a little, and also lobelia is very helpful in this type of thing. But the most helpful thing was a combination of calcium and magnesium, made into a drink with a little apple cider vinegar. It has to be the right amounts exactly because if not it won't work. Email me and I'll give you the recipe. As soon as i drank this I went and threw up for a while and then felt 99% better - then went right to sleep.
So there is help for this - I've been through it! Please email me if you need any more help... email@example.com.