“Why have I lost my appetite? ”

I haven’t really eaten in three days almost four days. Every day I take two nibbles of food and I have a loss of appetite the rest of the day. I’m not pregnant because I’m currently on my menstrual cycle. I have pain in my stomach that is not persistent but at times can be pretty painful. It’s causing me to be a jerk to people because of the pain. I was in the ER two months ago which I had blood work done and the doctor said everything was normal. I am currently not on any medication and I don’t have any health issues besides being overweight and I had pretty severe asthma when I was a child. Other than that I’m not quite sure what else to tell you. It’s not all the time I feel bad though, just at times where I’m offered food or I feel really hungry to the point my stomach is growling but I still can’t eat. At times today, I started feeling a little weak so I got down a meal replacement, I’m drinking well over a gallon of water a day because I work a physically demanding job.