As usual, Drnee is right on the money. The most probable cause is dehydration, and my two cents worth is to insure you take in electrolytes, such as Gatorade, along with water. You need to have what are called "orthostatic vital signs" taken. These are easy to do and a simple search on the internet will bring you up to date. Orthostatic vital signs are useful is identifying dehydration. The higher heart rate is usually compensatory, maintaining oxygenation with a depleted blood volume. The alligator-in-the-bathtub question is why you are dehydrated? Diabetes is one possibility. Anemia can be a contributing cause. You need to get a hematocrit. As Drnee stated, your blood pressure is a tad low, but not anything to go to general quarters about. Get yourself a good blood pressure cuff, take Drnees advice, and after a month of at least 32 ounces of fluid a day, plot the improvement.
Hi
Welcome to the forum!
A BP of 102/79 is on lower side but not dangerously low. A heart rate of 91 is on the higher side. An ECG and an ECHO should be done to rule out heart condition. Anemia and dehydration can also be the cause. Take more water and see if this helps. Please let me know if there is any thing else and do keep me posted. Take care!