I felt the same way as you describe, with vomiting and unable to keep anything down, around the 20 week mark too. I ended up spending 2 days in hossy on a drip to rehydrate me. BP was fine, and tests came back clear for infection etc, so I still don't know why. My ob said possibly pregnancy sickness, overtiredness or an earlier virus may have laid me low. I still feel dizzy if I do too much, or don't eat often enough (back to the morning sickness routine from the first trimester!)
HTH and hope you feel better soon. I'd suggest a check up too, just to be safe.