I'm not sure when her accidents started – if it was last year when you took her to the gastro Dr., then he may have ruled out encopresis. But if the problem developed some time after seeing him, she may have developed encopresis, which is not uncommon in the potty-training years, and often evolves after the child has had a painful bout of constipation and begins withholding.
The condition is caused by a backup of stool in the lower colon, which eventually numbs the local nerve endings so the child may not even be aware of a need to pass stool. However, softer, fresh stool squeezes past the blockage and leaks out, causing "skid marks" or small accidents. There are also emotional implications, because the child may feel embarrassed or ashamed, and not know how to deal with it.
Again, the kidlet may not realize she has to go, but may only become aware of passing poop afterward, by smell, irritation, or in your daughter's case, she may notice when she farts. She may not be able to help the situation, because the local numbing of encopresis makes that impossible. (In some kids, the numbing extends to the bladder and makes pee accidents happen, as well.)
So, if all this has become an issue since last August, I'd check again with the gastro doctor, or perhaps take her to someone who specializes in pediatric cases. If it is encopresis, she may need an extended period of stepped-up treatment with Miralax and/or Benefiber, and possibly enemas, that allows her to get completely cleaned out, shrinking the colon and allowing the nerves to function normally.
If that turns out NOT to be the problem, here's a very specific program that makes a lot of sense to help kids take responsibility for their own elimination: http://www.rogerknapp.com/medical/pottytrainingrefusals.htm
Wishing you and your daughter success!
Google encopresis for lots more information.