I'm sorry I don't know the answer to this. I don't experience a nervous stomach in particular. I just have this constant churning sensation throughout my body 24 /7. I don't know if there are any things one can do specifically for these kinds of nervousness symptoms. I think, perhaps, short of medication (which I'm not on, but have considered) the remedy may be simply things like physical exercise, meditation & whatever else one can do to assuage one's general level of anxiety. I presume, though, these sorts of things are not "in-the-moment" sorts of remedies. They have to be pursued consistently day-to-day, week-after-week in order to be of noticeable benefit.

Here's a link to an article, from PC's archives, I stumbled onto a day or two ago that offers 3 stress-busting tips. I haven't tried them (except that I have done number two for quite a long time in the form of what is referred to as "compassionate abiding".) So I can't necessarily speak to their efficacy. But perhaps they can be of some small help:

3 Stress-Busting Tips to Free Your Mind