I had stomach issues that started several months after my concussion. I was eating well in general but had a stretch of a month or so where I wasn't eating as well or getting as much fiber. The problem then seemed to recur intermittently over the next few months, and was really annoying and frustrating at times. This symptom seems to have largely resolved (touch wood), although I still watch what I eat.

Things that helped: moderating the amount of food I ate, while still trying to maintain a good diet; temporarily eating more bland foods (and fewer irritating foods - you can google lists of good/bad or more/less digestible foods); drinking a reasonable amount of H2O; making sure I got enough fiber (without overdoing it); chewing my food properly; and adding a good probiotic yogurt (BioK yogurt - Canadian company). Getting some modest exercise (even slow walking) every day also seemed to help.

As my stomach improved, I was gradually able to start adding back some of the foods that I had to cut out temporarily.

I think the lessening of this symptom was also simply a function of time.