Lately, I've been off with an upset stomach. I wake up to find something lying heavy on it. This symptom began to appear in my early twenties. I was working for a cram school on a part-time basis. This job could have been, as I look back now, something called "Black Baito" in Japanese now. When you are involved in it, what makes things worse is the fact that no one can get away from it, because if you do, those you left will suffer more due to the shortage of manpower. This was exactly the situation I was in back then. Stressed out, my stomach often began to ache, which I had never experienced before. Now, though I am free from such kind of part time job, when I am distressed, my stomach begins to hurt. Or better, stomachache tells me that I have been on the verge of being stressed out. I have a paper to write which is due on June 30th. This paper will be my first published work through a peer review ( though those who judge whether it is worth publishing are far from my peers). As is often the case with a chronic disease, I have got used to it. I seem to be able to go on with this stomachache, though it really hurts.
フレーズ（Oxford Collocations Dictionary For Students of Enlgishより）
お腹の調子が悪くて元気がない be off with an upset stomach