What is it called when you feel 'discomfort' that is not pain, is a bit like nausea but only in the lower abdomen ? the medical term nausea seems to be limited to the stomach ?

Choosing words: to describe symptoms that are precise and meaningful to most people is challenging. I think the word you chose--discomfort--would apply here. You are correct--nausea really implies a sensation in the upper abdomen, an area we call the epigastrium. Queasiness might be a good word for that same feeling--on the verge of vomiting--when perceived elsewhere in the abdomen. Folks will know what you mean!