I was out walking with my little dog about 3 weeks ago when a frisky dog jumped upon her rear end, only for a few seconds( 10-15 at most) before I could shoo him away. It was only afterwards that I realised she might have been in season at the time , hence his behaviour ( her first season - not even sure if she was and there was no spotting). I am worried- could she be expecting after such a brief encounter?

Yes, she could be.
Get her to the vets and get their advice. If this is her first season and she is under a year and a half, she is too young to have pups. Would be better for hwer to be spayed. It would be better to have her spayed than putting a pup thru a preganancy and mother hood, and bringing more cross breds into the world. :(
HTH
Lea :)

there is an injection the dog can have if you don't want to have her speyed called alizin it aborts any foetus not a good idea for a bitch at its 1st season though really but ask yer vet