is this a sign dehydrated??

So I have had times where I would all the sudden feel really dizzy and I loose my eye vision and I can't hear and I won't be able to figure out what's going on.. sometimes I feel like I black out for a couple seconds because then I find myself sitting or something. Today I was at the store with my SO and all the sudden I started to feel dizzy and my vision went completely black like I had my eyes closed I started not be able to hear him and I had to sit down on the floor... and then I think I blacked out because all the sudden I was closer to the store doors still on the ground. I was very very shaky. This is probably almost the worst one I've had... he was going to call the ambulance but I soon woke up from the black out and said I needed water. I was totally completely out of it.. is this a sign of dehydration? Soon after I said I needed water I was drinking a cup of water and slowly my vision came back and I started to feel better. I'm usually always drinking water, but I guess today all I drank was some milk in the morning and totally forgot to drink water. It was actually pretty scary because I had no idea what was going on. Any advice or opinions would help!