Thank you for your query at DoctorSpring.com
I understand your concern.
Just like most other antibiotics, Augmentin may cause some gastritis, and nausea, however, it does not cause bleeding in stools.
The dark brown color indicates gastritis and you mentioned the BM was loose. So it could be a different infection leading to acute gastroenteritis.
I will advise you to take Cap. Omeprazole 20 mg twice daily, once before breakfast and once in the evening.
If the dark brown stool persists, please revert back to me.