hey there. Sorry to hear about your experiences.
well, it hard to say by your description, but you could be having panic. Panic can show itself in many ways, and the feeling that something bad is going to happen is definitely one.
you can read more about them here:
Try this too, this help me. Try take notes when it happens and try and see what happens first. Like the second you have the feeling of doomed, write it down. Does the rollercoaster / heart drops come after that? Does it come before that? It will be interesting to see which events lead to the next and maybe there is a way to catch it " I feel that doomed feeling again" and do something to alter the coarse of the physical symptoms.
For me, and i think a lot of people, the order of events goes something like this.... There is a thought "I'm doomed" it can be totally subconscious for the reasoning behind it, but for whatever reason you have this thought... That then sparks a reaction in the form of an emotion... " I feel scared"... That fear than translates into a physical symptom " heart dropping into throat" which than leads back to the doomed feel, but this time its " Something bad is going to happen"... It becomes a cycle, but usually it all happens at once, so fast, or it doesnt really happen on a conscious level so it hard to break the cycle. Usually I end up having a panic attack, but i am getting better at controlling them, or at least recognizing them earlier on... Sometimes not though...
Anyways. My doc once told me " The second you feel like you are having a panic attack, write down what you are thinking" (because i was waking up at 5AM with attacks every morning and didnt know why). When i tried this it stopped the cycle some how. I got up to write down what i was thinking, and i ended up writing a lot of stuff that was just anxiety talk... But it did something to remove me from that panic onset... Also, exercising has helped a bit and the some other techniques that help me. Like riding my bicycle. Some days its so bad, that i'm scared to leave my house... But i tell myself " its ok to have this feeling and i can turn around and go home at any time..." I've never once had to turn around and i've always felt better after...
Sorry, this was way more than you ask for, and not even what you asked for, but maybe it will help a little...
Take care and keep us posted!
Anxiety and Panic Disorder.
"Breathe! You Are Alive"
- Thich Nhat Hanh