Why do I get awful stomach aches after working out...

[Bicoastal]

...having not worked out for the last few months?

[LSdemocrat]

1. Make sure you have the right balance of food/hydration in you before you exercise

The only thing I can think of is you either have too much or too little food in you before you workout. Ideally you should have something to eat, preferably something with some amount of sugars so that your body has fuel, at least 20-30 minutes before you exercise. On the other hand, you also don't want to go straight from having a full meal immediately into a strenuous workout. Also make sure you are well-hydrated before and during exercise.

If it's not a food and/or hydration issue, and you have serious pains working out, you really should see your doctor.

[BrklynLiberal]

2. If you are sweating alot, and thereby losing electrolytes that could be a problem.

Electrolyte loss can cause cramps.
You might try something like Gatorade to replenish the minerals.

The above post has some very good suggestions.