tips on lowering my heart rate before donating?

i've donated successfully 6 times, but i had to take a break for like 3 weeks. last week i went in to donate again, and everything was fine except for my heart rate, which was at 117. i waited 10 minutes and they rechecked and it was still too high. i've had this happen only once before, probably after my 3rd-4th donation, but then i went in the next day and it ended up being fine.

this time however, i went back the next day and it was too high AGAIN. it was at like 118, and then when they rechecked, it was 104. i haven't really changed anything about my routine, usually i go around 11am, stop at 7/11 and eat a danish + drink gatorade on the way, and everything's fine. the times i went last week i didn't stop on the way, but i ate a lot about an hour before i left.

i'm thinking maybe i just need to go back to eating something right before i go in, but idk does anyone else have this issue? i'm only 18 and i don't have any health issues, so i'm not sure why my heart rate has been so high when i go in. i'm never nervous to go in, since i'm used to it now and the thought of getting money immediately calms my nerves LOL but my heart's never beating that fast any other time (i checked my pulse about half an hour after i left and it was like 85-90)