A single accidental dose of Benadryl while breastfeeding is generally considered low risk for a baby this age. Only a small amount of it passes into breast milk, and one occasional dose isn't expected to cause harm.
Next Steps
Watch your baby over the next day for unusual drowsiness, poor feeding, or trouble waking for feeds, though this is uncommon after just one dose. There's no need to stop breastfeeding or discard milk for this.
Health Tips
If you find yourself needing this kind of medicine regularly, please check with your baby's paediatrician first, since repeated use is a different situation from one accidental dose.