trouble breathing in air

i have one question: i had my first race today, 32 miles, and with a loop we had to do 4 times with one killer hill.. on the 3rd and 4th time of that killer hill, of course i was dead tired, but i felt like i could have pushed a little bit more if only i could get more air! during the hill, my heart rate never went much higher than about 185 and found this weird since my max is somewhere in the low 200s, and the highest i have gotten it too recently was 202(prob. my max or close to it.) (by the way i'm 20years old.) and the weird part was, that on the 3rd and 4th time climbing that hill, it felt like i couldn't breath in enough air! it literally felt like my throat shrunk and i had a hard time breathing and getting air in! any help with that ? why it happened? etc..? Thanks.