Why do i bloat up at night and look like i'm pregnant, but look skinny in the morning?

ok here's the deal i weigh 125 and am fairly thin for my 5'2 frame, but at night my belly swells up and look like i am 8 months pregnant. i have had trouble with my bowels for the last 15 years or better, no matter what medicine i take or what foods i eat i still have constipation problems i've seen several dr's about this and they all say they same thing, eat more fiber, take a dietary supplement, drink more water, exercise more, i do and this only seems to be a temporarily fix. any ideas of what this may be? i've already been diagnosed with IBS and take medicine for it, but that only helps half the time and when i go i see spotting of blood.also when i wake up in the morning my belly is flat again and in addition to my belly swelling so does my feet and ankles