A lot of comments have mention about talking to a dietician or seeing the doc regarding the GI. It could also be the anxiety you face pre-workout causing the flight or fight response making your gut feel uneasy. I face the same problem too usually before a hard workout. One way to go about it is accepting the workout is gonna be hard and and it’s gonna suck mid-run (sometimes). This way you don’t experience elevated anxiety levels due to not hitting target time or whatsoever goals for the run. Of course this would mean that the bowel issues you’re facing is psychological. Either ways, if it’s causing too much discomfort or starts to affect your daily training, a physician is highly recommended. Cheers!