Is this likely to be gastroenteritis if I have achy bowel and softer stools after pigging out on carbs?

Agree w Dr. Huilgo. Gastroenteritis symptoms may include: diarrhea, nausea, vomiting, abdominal cramps/pain, headaches, muscular aches & pains as well as fever (low grade). Bloody stools are not usually associated with gastroenteritis. According to the CDC, viral gastroenteritis usually has a duration of 1 to 10 days (depending on the specific virus). The average is 1 to 3 days.
No. "pigging out" sounds as though you know the source of your problems already. Gastroenteritis implies infection.