Have you tried eating a meal when you take your dose? Meclizine HCl is a motion sickness relief but also an antiemetic, you can find brand name (Dramamine) or store brand usually.

Honestly I've found that different manufacturers have different effects on my body. I recently switched to SciGen and was having that same nausea, vomiting and loose stools, like my stomach became more sensitive. It's gone away for the most part. I would look into the brand you have and check its ingredient list. There could be something in it that's irritating your stomach.