I had my leg slightly bent and when i straightened it it cracked with slight pain what could this be?

Joint crack usually: When we go from sitting to standing, or standing to sitting and start walking sometimes we hear our joint crack. This is normal, and not normally a concern. However if you're experiencing pain when doing so, you need to call your doctor. They can check you closer on physical exam reproducing the same sound. This will make you worry less, and feel better knowing that you've had it checked out.