You can take Miralax everyday or every other day.
Eating lots of veggies, fruits, and lean meats (fish, chicken) can help. Fast foods, gluten, dairy, chocolate, or breadth foods can all be constipating. You might want to try keeping track of what you eat during the week to see if there are certain foods that cause your stomachaches.
I also buy probiotic powders and pour them in my almond milk or yogurt in the morning. Probiotics are natural, good bacteria for your stomach.
Going on walks or doing some type of daily exercise can help keep things moving, too.
And drinking lots of water throughout the day helps all the fiber / food you eat to go through because even fiber can be constipating in large amounts.
As a fellow Aspie, our stomachs can be naturally more sensitive to foods than other people's. Hope this helps you!