About a year I had long break from doing any type of fitness (I pretty much was bed ridden due to depression, lack of energy and just no will to do anything, but managed to keep a fair amount of fitness ability), and about 5 months ago (maybe 6) I started doing more things again, mainly little things like going out to hang out with my horses, and going to a therapist for a bit. Then around 2 or 3 months ago I decided to go to a fitness trainer to get myself even more fit for my horse riding. I've been held back quite a bit by an issue, which seems to be heart-related. What tends to happen is normally when I do something strenuous (but it also happens when temperature switches or air just becomes more humid) I'll start feeling my heart rate become quite rapid and heavy feeling, then I feel dizzy and if I'll quite quickly feel sick and have to throw up or at least dry-retch. The doctor I went to thought it might be something to do with my thyroid glands so took a fall blood test, everything looked normal, my trainer thought it might be anxiety but it's happened at times where I'm at my calmest (around horses), and my parents thought it might be because I'm unfit, but after doing training and my fitness is pretty much back up to where it was before my break, it's still happening and hasn't improved at all.