Re: Left abdominal pain and rectal bleeding.
When I see left abdominal pain, the first thing I think of is diverticulosis/diverticulitis. Before mine was diagnosed, for many years I had eaten more fiber than average (15-20 grams) and was regular at least once a day, but later found out there was a genetic component, so you never know.
I'm glad you have the appointment so soon with the GI . . . sometimes it can take a month to 6 weeks around here to see a specialist unless your situation is acute. There are various possibilities, so it will take the hands on exam to find out for sure.
If you start running a fever, get medical attention as soon as possible.