Can post nasal drip make me feel sick? not just nausious but really run down.

Possibly but: Many people have post-nasal drips and only rarely do people get really sick from it. I would suggest that you talk to your doctor about this since i'd doubt that post-nasal drips are causing much of your problem.

Possible: Alot of patients with post nasal drip will get an upset stomach because of the excess mucous in their stomach. In addition, if one has a cold or allergies causing the drip, one would expect some sleep disturbance b/c one's nose is not functioning properly b/c of the congestion and drip from the cold/allergies. This could contribute to the feeling of being run down.