I stopped cetirizine after taking every day for a year, and now have sour stomach/nausea. Is this withdrawal? Is there harm in taking it long-term?

Not expected. This is not an expected reaction from cetirizine which has no known withdrawal symptoms. There is also no study showing hard when taken long-term. You may just have a coincidental viral infection. See doctor if the problem lasts more than a few days or gets worse.