
Be patient.
If she hasn't been outdoors for a while she is scared of the big outside! This is why pups need to taken everywhere possible in their first few weeks of life. Are you taking her (carrying of course) round the town, to the shops, past the railway station, past busy traffic, to meet horses, cows, sheep etc - to see old people, young people, children, prams, pushchairs, bikes - people with hats, people with beards, wheelchairs - you name it, the pup needs to have seen them, in saftey, before it is 16 weeks maximum.
When you're in your back garden, just take her out, put her down beside you and ignore her. Don't fuss her and 'reassure' her - you will only be rewarding her nervousness. Potter about the garden and let her get used to it in her own time. She'll be as bold as brass in a few days!
Good luck.
:)