Honestly it really comes down to your diet. Take a look at what you had to eat the night before and log which days you had the most trouble on. Stay away from too much fiber as it only adds to the "runners trots". My last Half Marathon I did I ended up walking a whole lot due to this same thing. It didn't hit me until around mile 10 though. I was running around a 7 flat pace until the 10th mile and ended up finishing with a time of 1hr 48 min. In training I went 1hr 35min so I was a little on the upset side. I'm right there with ya and people like you and I have to really pay attention to what works with us or against us.