Try taking smaller hits instead of huge bong rips. Sounds like you could be swallowing some air and that can make one feel sick to the stomach. Exhale your lungs empty then take a light hit from the bong. Make sure your lungs are not filled with air when you take a hit.

Or, as someone said, maybe you're swallowing after you take a huge rip, either the smoke or again, air.

Use clean water everytime and change often.