I get upper stomach pain (GERD-like pain) after a bowel movement. Without freaking me out, what could it be?

Unfortunately, the lack of nerve endings within the abdominal cavity makes locating the malfunction very difficult. You might be suffering from something as simple as indigestion or something as serious as heart problems.
No doubt you are medicating yourself, but my advice is to go to a doctor, especially if the pain persists.
In the meantime, try taking one or two tablespoons of liquid Mylanta in the morning. If you are developing a gastro-intestinal problem during the day, this should relieve some symptoms.
Watch your diet carefully: try to curb your fat intake and limit your fiber to a reasonable level. Here you are trying to eliminate difficulties with the gall bladder. Too much fiber often causes gastric distress as does too much acid, even things such as orange juice and coffee. I suggest omitting roughage such as salads and stick to a relatively bland diet, at least for several days.
Make a list of what you are eating for several days and look for a change. Also, look for blood in your stool: this will appear as extremely dark discharge. If you see this, consult a physician immediately.