Abdominal Bulge That Becomes Pronounced After BM, Not Tender Or Painful, Ultrasound Normal. Any Ideas?

Question : I have a strange lower abdominal bulge that becomes extremely pronounced after a bowel movement and then slowly begins to even out, but never completely goes away. It is not tender or painful at all. If I push on it, it stays the same. It has been 2 months since I noticed it. Its right under my bellybutton to the left. It almostis appears as a wave. There is a small indent above my bellybutton and then the bulge. When I lay down its gone. When I stand and fully relax my abdomen , you can see it. When I lean back slightly, my abs tighten and it disappears. Ionly weigh 115lbs so its very noticeable to me....I've seen my dr but usually its not horribly noticeable unless I've just had a bowel movement so my dr has never really seen how bad it can look. She referred me for an abdominal ultrasound which came back "normal" . I also have slighter rightly hip/back ache but I don't know if the two are related. Any ideas. Im not taking any meds.
My stool seems a little looser than normal but no diarrhea . I have had slight constipation but nothing chronic. Any ideas?????