I've Had A Fullness In My Upper Abdomen All Day

I've had a fullness in my upper abdomen all day like I've drunk too much water or like I'm in the latter stages of pregnancy and I haven't eaten all day. My back is tight as well. I suffer from IBS with constipation and also GERD and night before last I thought I was having symptoms of a heart attack or some sort of anxiety attack so I took a laxative thinking it's probably GERD and yesterday I finally got some relief and felt somewhat better, but as today has progressed I'm feeling more pressure with a pain (mostly if I push on it) in the upper left stomach right at the rib cage. I'm also feeling stress in the neck. I did go to the doctor a few days ago with what I thought was a sinus infection so I was put on a z-pack and prednisone which I started yesterday. I just need to I should worry and pay attention to other signs. I have an appt for a complete physical this Thursday.