Luke, I am sorry that you are having this problem with Bella. Diarrhea can be caused by many different medical conditions. It will be difficult for me to determine the exact underlying cause without a hands-on examination and some testing. But, common causes are dietary indiscretion, ingestion of a foreign body, infections, or even part of a systemic disease to name a few causes. This is what you can do:
1- Pepto Bismol 1 teaspoon every 6-8 hours for 1-2 days. You can buy Pepto Bismol over the counter in any pharmacy (chemist).
2- No food or water for 6-8 hours, need to give her gastrointestinal tract time to rest. Then, continue on the diet of rice with boiled boneless chicken or ground beef until the stool is back to normal then wean her back into the regular diet.
3- Supplement with a probiotic, these are friendly bacteria that help with regularity. You can buy this over the counter in any pet store.
4- If after doing the above you do not see improvement within 24 hours I would have her checked by your family veterinarian for a hands on examination. Discuss with your vet about doing some testing like fecal analysis, blood work and x-rays as an initial screening test.
My goal is to provide you with excellent service - I hope I've provided the information you were seeking, if you feel you have received anything less, please reply back as I am happy to address follow-up questions. Kindly rate me when you are done.