Rita, first off-congratulations on your PR and completing the marathon. Feel proud of those two accomplishments. Taking into account the hills on the course your miles splits were great. Had it not been for the lost time during those 4 bathroom breaks your BQ time is right within your grasp.
Since the problems don’t start until after you’ve been running for awhile you can rule out nerves or anxiety. You’ve probably all ready done this, but have you looked at your diet even 2-3 days before the marathon?
For many runners the digestive system slows down during the marathon because blood is diverted to other areas of the body that need it. Not in your case.
What/when did you take any gels? How about hydration?
Just wondering what prior to mile 14 would have triggered the GI problems.
You’ve mentioned the great support your family gives you, you know they have your best interests at heart. Hard to see someone you love put so much time and energy into something and then not have it meet expectations. Concern is natural.
Take some time to recover from this race, think about your running and where you want to take it. You seem like someone who enjoys a challenge. I have no doubt you’ll get this figured out & be working towards that BQ again. Oh, take care of those blisters.