It could be many things, but is particulary dangerous if you're driving
Are you on BP meds or is that a natural reading of 99/55? It is low but not horribly low. You mentioned that you have started "exercising and eating right". If you're not eating enough, it could cause your "sugar" levels to drop which can cause dizziness. You already know to drink enough fluids, especially since you are excercising. Make sure you drink or eat something "salty" if you've been sweating a lot. Most of those sports drinks contain sodium and other electrolytes. If you continue to feel dizzy, it's best to get a check up just to be on the safe side. Take care!