Black fly bites can indeed cause swollen lymph nodes and diarrhea in some cases. It is important to monitor your symptoms and stay hydrated. Eating light, easy-to-digest foods like crackers, soups, and eggs may help. Avoid heavy or sugary foods like red meat, undiluted fruit juices, and milk. If your symptoms persist for more than a week or worsen, it would be wise to consult a doctor for proper evaluation and treatment.