M.O.
I don't think she is getting enough hind milk. That is the fattening good stuff! Idealy she should be nursing for between 10-20 min on each side (it's okay if she doesn't eat from the second side). My 5mo. old rarely exceeded 15 min. after she was 8 weeks. Anyways, it is highly unlikely that your milk is giving her a tummy ache. It's more likely that formula will do that. She may be sensitive to something in your diet (milk, cinnamon, tomatoes...) but remember that women in India and Africa nurse their babies for years without issue and spicy diets, so that is less likely too. It sounds to me like she is still hungary.
Giving her formula will not encourage your milk production it will decrease it since she is not nursing as long or as often.
The protiens in breast milk are much more easily digested than the longer protiens in formula, and that is why formula keeps them fuller for longer. However, the baby does not get as many benifits becasue they have to spend more time breaking stuff down before their bodies can use it.
It will get better. You have to remember that she is barely a month old. She will be nursing often for a while, and then the time between feedings will increase, I promise.
Mother's milk tea is wonderful to increase your milk production.
I would suggest giving her bottles of breast milk and see if that helps. Pump till you are empty and offer her that. Then she will be geting hind milk. If she is still fussy check you diet for common irritants. If that doesn't work, try gripe water, she might just be gassy.
Have you seen a lactation consultant? They would be a great resource for you. "The Nursing Mother's Companion" would be a great help! This book adresses every issue that a nursing mommy can have (answers all the questions you didn't even know you had) and supports you in your quest to give your baby the best. I would invest in that as well!
I'm not sure where you live, but you can start going to La Leche Leauge meeting too! All the kidlettes can come along.
My advice is to not suppliment. At least not yet. Not until you have exausted all other avenues. Women have been nursing for thousands of years with no option for formula, it's in your genes! You can do it, you might just have to adjust something to make your little girl happy after her snacks.
I could go on all week about the things you should try and do instead of formula, but you have kids to attend to, I'm sure.
There are lots of nursing moms, such as myself, on mamasource and would be willing to answer any question you might have.
Keep going with the nursing, you can do it!