Sorry to hear the race didn't work out as planned. There are a couple of other possible causes to consider:
1. Illness. Are you coming down with something?
2. Digestion. This is gross, but workouts and races can go bad when you're unconsciously clenching abdominal muscles to keep in stuff that wants to get out.
But it could be something else, and the longer the race, the more things that can go wrong.
It it was me, I wouldn't jump right back into a half marathon. 3 weeks isn't a lot of time to recover, and you probably want to get a better sense of what went wrong and where you are now. What about running a 5K or 10K (or more than one) to get a better sense of your overall fitness first? Plus the shorter races take much less time to recover from.
Do you have any HR data from the race that you can compare with workouts, or HR data from your 7 miles tempo? How much are you straining at the end of that workout? And how did you measure the distance?