My HR drops after drinking water while running... why? and is there something more I can be doing or something I am missing

In the past, I hardly ever drank water on my long runs. The few times I have, I didn't have a smart watch and/or a controlled environment to see the effects.

In July I started running in zone 2 for 90 minutes 4-5 times a week. The past few weeks Ive had to train inside on a treadmill due to weather and I started taking water with me to the gym. One thing I have noticed is after drinking a few ounces of water my heart rate seems to go down 3-4 BPM. 5-10 minutes after I take a drink of water my HR creeps back up 3-4 BPM. Perhaps its a dumb questions but is there a way to keep my heart rate down for longer. (gatorade or another drink)

I try not to drink a lot before I run because I have an overactive bladder and by the end of my 90 minute run I need to pee really bad. I try to drink plenty of fluids 120-90 minutes before I run but maybe I'm not staying properly hydrated before the run.

I am preparing for a 15.5 mile race in May and I dont want to have to get a drink every 5-10 minutes to experience the benefits of lowering my heart rate 3-4BPM.