Why do I have rapid heart rate and dizziness after doing strenuous activity?

Hello doctor,

About a year ago, I had a long break from doing any type of fitness. About five months ago, 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 two or three months ago, I decided to go to a fitness trainer to get myself even more fit for my horse riding. I have been held back quite a bit by an issue, what tends to happen is normally when I do something strenuous (but it also happens when temperature switches or the air just becomes more humid) I will start feeling my heart rate become quite rapid and heavy feeling, then I feel dizzy (my head feels really weird) and I will quite quickly feel sick and have to throw up or at least dry-retch. The doctor thought it might be something to do with my thyroid glands so took a full blood test, everything looked normal, my trainer thought it might be anxiety but it has happened at times where I am at my calmest, and my parents thought it might be because I am unfit, but after doing training and my fitness is pretty much back up to where it was before my break, it is still happening and has not improved at all. I have always done sports for most of my life before the break, the main two being athletics and horse ridings (though I only do horse riding now as well as fitness training).