[cpowerg]

trans started slipping. does anyone know what's wrong?

[richlander]

You are a little vague on details, is it automatic or manual. If it's automatic try checking your fluid level when its hot, it may be low. If it looks brown or it smells burnt you need to replace the fluid in it.
If it's a manual, your clutch may be going out. In both cases, a lot has to do with your driving style, mileage and maintenance you put in it.