A loose connection somewhere. Possibly inside the light bulb in which case you cannot repair it.\
With the power off check the inside of the socket go see that the tab at the bottom sticks up a little to be sure it puts some spring pressure on the bottom contact of the bulb.
Stick to your lawn watering schedule until it really starts to pour. After the storm you have only the same number of rest days you always had and then you need to start watering again.