Can spondylolisthesis worsen even if I have no pain?

Yes. The most common type of spondylolisthesis will rarely progress to a severe slip. Most of the time, the progression stops when a person reaches adulthood. The other common kind occurs in middle life and beyond, and rarely progresses beyond grade 1 or 2 (mild). The process of slippage is not something people feel.
Sit up straight. The lower spine can slip forward if you are not careful with posture. Also, lose weight and strengthen those stomach muscles. The spine needs good care all of your life - whether it hurts or not!