It's hard to offer an opinion without seeing the incident or a good video. Even then, if we had the video there would probably be a lot of discussion about the event. Some of the opinions that might be offered:
1. Attacker lost her footing and subsequently tripped or unfairly charged your DD such that both ended up on the ground - should have been DFK for your team
2. DD fouled the attacker causing her to go down - PK but advantage immediately accrued so Goal
3. No foul in initial contact but DD fouled the attacker by using her legs to hold down the player - PK but advantage immediately accrued so Goal
4. No foul in the initial contact, dangerous play by attacker - IDFK for your team
5. No foul in the initial contact, no dangerous play - Goal (Your situation)
The referee was there, saw it all, and offered his opinion, i.e. no call. His game, his call, his reputation!