The ER is not the place to go for something like this. They take care of emergencies and have an examine and move out mentality. This isn't an emergency but a problem going on with your health that requires a regular doctor. You can see if your doctor can move you up (they have cancellations and a lot of times have space blocked out for an acute situation that people have to be seen for that day, example . . . if you have pneumonia, you can't wait until a week to get antibiotics, so when sick, they get people in . . . you must not have sounded like it was necessary right away, so you can call back and up the ante so to speak and see if you can get seen same day or next day).
I had an ulcer that created symptoms like you describe. It was an ulcer in my small intestines. Have no idea if your issue is the same but they can run some tests on you (they found my ulcer through an endoscopy). So, you see your regular doctor and hopefully they will provide an answer or they will refer you to a gastroenterologist.
Oh, and on the note of ER, I have found for a sickness or issue like you are having, urgent care is a better option. Urgent cares these days see regular patients too so have that mentality. And it may be cheaper too (my ER copay is the most expensive of all my copays). good luck