I used to experience this, too. short 5-7 mile rides at moderate pace would have me urpy and nauseous for hours after when I was starting out.

What's helped for me is a small, light snack before, eating little and light during, only when necessary, drinking a lot of water w/ or w/o electrolytes, and then waiting maybe half an hour after riding for your heart rate to find normalcy before eating?

When it happened to me, I also got a strong ammonia scent through my sinuses. Not sure if/how that's related; just a thought.