My Nissan sentra 2.5L engine is smoking white smoke after i changed the catalytic converter what could be the problem?

When you say "white smoke" what do you mean? Is it... you know, like... steam?

Understand that the products of engine combustion are carbon dioxide and water vapor. If the weather is cold enough and the exhaust is cooled by a cold exhaust system, you'll see water vapor out the tailpipe. Once the exhaust pipe warms up the water vapor will still be warm enough as it exits the exhaust pipe to not be visible even though it's still present.