Feeling of fullness in mid-upper left abdomen--any idea what could cause this?

I have Crohn's, and just got put on Prednisone to control my disease about 10 days ago (I have begun a slow taper down from 40mg and am decreasing 5 mg a week, currently am at 35 mg a day). Across the last month and a half, I have begun to have this feeling of fullness, or like there is something there, in my mid to upper left abdomen. I had an ultrasound and it showed nothing (organs looked normal) and thus far there has been no explanation as to what could be causing this. My GP suggested it could be Gastritis and wrote me a prescription for Nexium which I have been taking for like 5 days.

So far I still have this feeling, I don't even know if fullness is the right word....it doesn't hurt it just feels like something is there. It's like just below my rib cage. If it was Gastritis wouldn't I be better by now? I suppose it could be just Crohn's stuff, but the Prednisone seems to be keeping my other symptoms under control and the strange thing is that the majority of my inflammation is on the right and I usually feel Crohn's stuff in my lower right abdomen.... I felt like the feeling was going away and then when I flared and started the Prednisone, it kind of came back.

Anyway, I'm kind of freaked out about what could be causing this feeling. I see my doctor in like 3 weeks so I guess I will discuss it with him. I am worried it's something like cancer or HSTCL....I guess maybe that would have shown up on the ultrasound. I guess I was just seeing if anyone has had a similar experience and, if so, what caused it.

This may seem overly simple, but could it be trapped gas?