Sexual activity, including masterbation, causes many changes in the chemicals in your body. All of which are normal and healthy changes. During masterbation, your body creates huge amounts of endorphines (which make you excited and alert) and hormones (which drive your sexual appetite). Sex and masterbation is basicly like exercize that feels really good. When you have an orgasm or exercize extensively, your body releases another chemical called seratonine. This is the chemical that makes you feel good. Seratonine has an almost sedative effect. Meaning that it's purpose is to calm, relax, and make you feel good. Most people who masterbate or have sex experience what you are describing. They become sleepy, lazy, and pretty much just want to lay around. Don't worry, it's just the seratonine. It's completely normal and harmless.
There are NO side effects to masterbation and it is actually good for you!