I Am 77 And My Blood Pressure Is Usually 109/60

I am 77 and my blood pressure is usually 109/60 heart rate 61, The only health problem I have is hypothyroid, I weigh about 138 lbs. workout with Silver Sneakers about 2 to 3 times a wk. I usually feel good, some times I feel a little lightheaded. I sometimes feel that I am dehydrated, I try to drink more water, I use a water Senetiva mineral water Filter could that be causing low heart rate?