I Was Mowing The Grass This Morning And Felt Dizzy

I was mowing the grass this morning and felt dizzy light headed came out in a sweet had to come inside sit down drink water and rest I still feel week and no energy , I am a very active persone have just retired ,I rang my doctor and made apointment for Monday , This is ant the first time this has happened have been to the doc many times no out come can you advise me