Antibiotics are often needed in young babies when an eye infection doesn't clear with drops alone, so Bactoclav was a reasonable next step. Loose, watery stools are a well known side effect of amoxicillin-clavulanic acid, since it also disturbs the normal gut bacteria. This usually settles once the course is done and doesn't usually mean the medicine needs to stop.
Next Steps
Keep breastfeeding or feeding your baby as usual, since fluids matter most right now. Don't stop or change the antibiotic dose yourself. Call your baby's paediatrician today to confirm whether to continue, especially since she's underweight.
Health Tips
Go in urgently if you see fewer wet diapers, very dry lips, unusual sleepiness, or if she refuses feeds, since these can mean dehydration in such a small baby.