I'm very worried about what this could be. I have a lot of things going on right now, I will try to summarize as briefly as possible:

I am 30 y/o white male. Have had occasional bad constipation throughout my life.

- Over the last several months, I have been feeling increasingly bloated. About a month ago, my stools started getting very short or narrow, and my bowel movements were very limited and always felt incomplete.
I went to see my doctor, and he gave me fibre supplements which have got my stools and movements back to normal, but the feelings of bloating, stomach pains, and incomplete defication all remain.
I am also experiencing nausea, which I have had occasionally for a while, but becoming much more frequent now. Same with lethargy.

Other things to consider: I have had bouts with anxiety and compulsive worrying in the past, and when I start becoming concerned, things usually become worse. For example, about a year and a half ago, I felt absolutely terrible, experincing everything from chest pains to rapid heartbeat to dizziness. After a number of tests came back negative, I started to feel better.
I get lots of fiber in my diet, although I tend to binge eat sometimes.
I have never seen blood in my stool and I don't think I've had mucus either, although I'm not sure what it would look like. My stools do seem to be lighter in color right now.