Be sure to clean your water bottle!

Here is a tip from About.com on cleaning your water bottle:

Cleaning Your Water Bottle: Both single-use and reusable water bottles should be thoroughly cleaned and dried between uses. Reusable water bottles generally have wider mouths, making them easier to clean. Dishwashing soap and hot water are acceptable to use for cleaning your water bottle. The risks of bacterial and fungal growth are higher if you use the bottle with a drink that contains sugars. Immediately drain, rinse, and wash your water bottle after your walk if you use it with sports drinks or juices.
Sanitizing Your Water Bottle: If you have visible bacterial slime or mold in your water bottle, you should sanitize your water bottle with a dilute bleach solution of 1 teaspoon bleach and 1 teaspoon of baking soda in 1 quart of water. Allow the solution to sit in the bottle overnight, then thoroughly rinse and dry the bottle before using it again.