Whenever I put my retainer on in the evening it feels very tight, especially around the front teeth. Does it mean that my teeth have shifted during the day? What is the recommended amount of time to wear a retainer? Also, is it ok to drink water with a retainer on?
I will answer your questions separately:
1) Teeth can shift on their own, due to the fact that they are supported by the periodontal ligament (PDL) that holds the teeth by their roots in your mouth, while still allowing them to move slightly. Think of it as a shock absorber.
Most movement of teeth happens due to pressure that is applied to your teeth (ex: biting when eating) but also when an orthodontist slowly moves the teeth into place by having the orthodontic wire push and pull your teeth in a certain way for a long time.
Wearing a retainer is important, to prevent the teeth from returning to the way they were before wearing braces. The periodontal ligament has a certain memory and will tend to "spring" back to where it was before.
2) Your orthodontist should tell you exactly how often and how long you should wear the retainer. Here is an example of what an orthodontist might tell you: "Some retainers are used full-time for the first 6 months; after that, the retainers are worn only at night, for a few years. Other retainers are worn full-time for about a week, and solely at night thereafter. Fixed retainers are normally kept in place for 5 years" 
3) Yes, you may drink water with your retainer in your mouth. Avoid eating or drinking anything else when wearing your retainer.
Some of my Sources: