Our body sometimes does weird things to us, sometimes to warn us of problems, sometimes just being a nuissance with no real problems.
I think I understand what you have been experiencing. I have felt something along the same lines myself, though, not recently. I also suffer from anxiety attacks with no real reason for them. Sometimes feeling great and then all the sudden start feeling anxious with no ryme or reason behind it. I found online a natural product called Amoryn and Seredyn. These two natural products have helped me tremendously, not cured me, but helped me a lot. Research Amoryn and Seredyn online for yourself, you may be able to at least reduce the anxiety fears with these products.
I think you maybe have been experiencing that first fear filling like one would if they were dropped quickly like riding a roller coaster. At least with the roller coaster, you understand why you feel that fear feeling, but, when it happens for no real reason, then, a person starts thinking of all kinds of reasons for that feeling, like heart problems which again is scary to think about having heart problems so, when it happens, this is usually the first thing people worry about is a possible heart problem. Then anxiety creeps into the picture about having an attack while around other people etc. etc. etc. the list can get quite large. I have probably had every fear known to man, and well, I still get my work done, travel, etc. etc. etc.
I think that the anxiety is simply a chemical imbalance, and the anxiety is related to that imbalance in the body, either we are getting too much or not enough of something that keeps our bodies regulated.
Hopefully this helps a bit....try not to be scared about the anxiety...also eat as much natural foods as you can possibly stand. Also, water, water, water. Keep your body full of vitamins and minerals.