Stomach Issues

Hello all, new triathlete here. Little about me, I was 260 back in February and I'm down to about 206 now. I'm following a Training Peaks Full Ironman plan and hoping to complete my first Half-Ironman next year and possibly the full in 2021 or if I feel like it, then in 2020.

I have read this article here: https://beginnertriathlete.com/cms/article-detail.asp?articleid=1240and I think this might be my issue. I have experienced stomach cramping twice since I started my longer training sessions. I cannot recount the details from my first one but the second happened about 12 hours later after a 1.5 hour run at a zone 2 pace. I did have about 24 ounces of Gatorade with ice that I slowly drank parts of the run. I ended up eating meatballs, green beans, and mashed potatoes after that and went to bed about 4 hours later. I have done a longer run at a much faster pace and haven't experienced anything like this.

I've been working out and not tracking all the calories I put in. I know I'm getting around 1800 and I do try to eat more veggies/meat/yogurt on days where I worked out over 500 calories.

Am I not eating right, I don't even know what fat/carb/protein ratio I'm in, is this a potential issue? Any advice would be greatly appreciated. I am considering seeing a nutritionist because I don't want to fail my first Half Ironman and destroy my body during the training sessions.