Quoting Sarah Latham:" My supply seems to be slowing down so my midwife suggested trying her last feed as formula"
A midwife suggested you feeding formula? That is odd for them.
To increase your supply you have to put the baby to the breast more often. Also, you need to make sure you are properly hydrating and eating enough healthy calories. You can't "diet" while you are breastfeeding. I promise you that if you eat properly and keep nursing more often your supply will grow leaps and bounds.
One more thing, it might seem like your supply is going down because you aren't leaking all over the place anymore. All it really means is that your body has adjusted to the needs of your baby and is not over producing anymore.
Yes, she could have a tummy ache from the formula.