How to lower my Pulse for plasma donating?

for some reason today my pulse was 103 today so i wasn't able to donate today first time iv ever had this issue the one thing i changed was the amount of water i drink before donating started drinking two 3 quarts bottle of water the night before and one 3 quarts bottle 1hr before i go the nights before i go i don't drink any sodas or anything with caffeine in it and just water to drink didnt start the water thing till last night is it possible its the reason