tap water-low blood pressure

I have been using spring water for a while along with alkalizing, and it has been going well all my symptoms of ill health have been improving steadily. A week ago I ran out of my spring water and have been using tap water one of my health issues that was improving -low blood pressure- is now back again, since I have a blood pressure monitor I have linked this downturn to the tap water. So tomorrow I will be stocking up on the spring water.
My question is this.. drinking the tap water did not effect my pH negatively, so what could it be in the tap water that causes my blood pressure to go too low? I also notice that a long hot bath will also cause my blood pressure to go to low.

Perhaps it was actually something about the spring water that helped raise my low blood pressure?

thanks!