Yeah. . . you bonked and/or got dehydrated (probably both). Did you get lightheaded, vision blurry or some such symptom. That's what happens to me. I'd suggest that you sign up for the saturday app- free for 2 weeks. It's eye opening how much nutrition you will need especially with a 1 hour-ish run after.

Is this your first IM? meaning first set of long bricks?

For example. Today, I rode 3 hours at slightly below IM pace. Ate 1000 liquid calories (in 72 oz). Ran a brick 4 miles (ate 100 cals/ 10 oz) and I was slightly under-fueled and got weak/lightheaded.