How did i get crabs? should i have looked for some signs on her?

Yup from her (him) : Crabs are human lice that infest pubic/anal hair or eyelashes. They don't invade & were passed down to humans from gorillas about 3 million years ago. You may not have seen any rash on her depending on their life cycle during your exposure. You would have had to use a magnifying glass to spot the eggs or the adults since they are only 1 mm long. A magnifying glass before sex is a sure mood killer.

Direct contact: Dr enriques provided an exceptional answer. Crabs (pubic lice) is usually transmitted through direct contact during sexual activity. There can also be vector transmission via contaminated towels, sheets or clothes.