First of all you need to understand the life cycle of worm. worms are present in the intestine of cattles they passed through their potty and if we ingest contaminated water or food. their eggs are hatched inside our intestine and we get the infection. if you are infected with pinworm how it can pass to your 3 month old baby through your milk. but it can pass if you do not maintain hygiene you can manually check your baby's potty or do the stool routine microscopy examination to detect your baby is infected with pin worm or not but as per my opinion if you breastfeed your baby there is less chances of your baby is having infected

Get her stool routine examination done

No,it won't effect the baby.