Water may not be enough. Electrolytes are important too. If you are sweating more than normal, your electrolytes might "be off" a little. Try drinking some Pedialyte - it's better for you than Gatorade. Or make your own rehydration drink: 5 cups of water, 1 Tablespoon of sugar and 1 teaspoon of salt. Add a pinch of baking soda. This doesn't taste the best - but it replenishes the electrolytes.
Wishing you the best.
You should make an appointment with your doctor to get your knee checked out.
You may have arthritic problems now in your knee. The numbness in the foot could be from referred pain from your knee or lower back, inappropriate footwear or nerve damage. Apply hot, or cold packs to the knee. Use the one that helps the best.
If the climate is hot and you work out a lot and sweat, you may require more than the 3 litres of water throughout the day.