Poor you and Tia.
Sounds like you were doing all the right things by keeping her on the lead but these things happen.
The same situation happened to my Mum's dog, she slipped from the car whilst in season and found a dog within minutes (as they do!) Mum took her to the vets and they spayed her the next day, obviously preventing the pregnancy continuing further.
If you definately don't want puppies from her I would say this is your best option.
Good Luck.