What discourages me

i been to a tournament recently. we reached the finals. there is a best of five team even with 2 singles then a doubles and den again two singles if needed. i hadn't played any singles in the entire tournament but i was really confident in the finals because i had played well in the semi finals. our team lost the first two singles , den won the doubles and the next singles. the score was 2-2 and i had to play the decider singles match. it was a best of five match. i wasn't against a very good player and i won the first set by 11-9 . den i don't know what happened to me after that. when the second set was going to start i realized that my heartbeat was rising and my legs didn't seem to make effective movements as i was able to in other matches.i just got nervous and then i lost the proceeding three sets under five and lost with a set score of 1-3 . it was really a shameful defeat as i was expected to win the match really easily. but i lost that last match and the finals. 

i just dont understand what happened to me suddenly and what was the reason that i came under pressure and this was not the first time it happened to me in such situations.

P.S. i dont suffer from any medical problems as such that could cause it. 

thank you, your reply would really help me since there is another tournament shorty and i need some mental preparation.