Weird event when mountain hiking...?

So today, I went cycling in a rocky and hilly state park. As I walked my bike up some super steep hills trying to keep up with my grandparents who were on horseback, I noticed that even though my heart was beating like crazy, I had lost the need to breathe. My vision started to become fuzzy and felt like I was going to faint. I just told myself to take deep breaths and eventually, everything returned to normal and we continued our adventure.

What happened????? Was it the sudden altitude change, because we were pretty high up. Thanks!!
 
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