Hypertension, Stomach Flu, Blood Pressure Up And Down. What Can Be Causing This?

I was diagnosed with hypertension a few years ago and was put on metapropolol succinate (50 mg). After I changed my diet and went on a low salt diet and eating much better, my blood pressure went down and the doctor lowered my med to 25mg. When I became ill recently with a respiratory infection and then stomach flu , my blood pressure fell low, and the doctor said not to take the med. However, when I came home from the hospital, I got blood pressure spikes and took the med unless my pressure fell low (under 100 systolic). I can t understand these up and down pressures. What can be causing this? I can t think of anything I am doing differently. I am supposed to be seeing my doctor again.