I have infrequent knee pain. is it bad to do threadmill if i have this pain?

Unlikely: If you have only occasional knee pains then i think it is unlikely you will do any damage to your knee by doing treadmill or any exercise, for that matter. Usually if there is significant pathology such as a torn meniscus, the symptoms are more constant and severe. Certainly if your knee pains become persistent or severe, then you should see a physician for further evaluation.

See below: As long as it is not a steep incline, you may do fine.

See below: Should be fine. Do not overdo it.