Congratulations on being sober for 10 months. It's really difficult to change your life and takes a lot of courage and discipline.
I think that the answer given by Bauercvhs is pretty right on. From what I've read, the standard fingernail test will evaluate use from 3-6 months past (i.e., not what's going on now). The tip of your fingernail is the oldest section. The newest part of your fingernail is near the cuticle. Assuming the test was taken from the tip of your fingernail, and if your nails are longer or grow slowly, it's possible this test reflects the THC in your system from several months ago.
As Bauercvhs suggested, for your follow-up test, if the sample will be taken from the fingernail tip, be sure your nails are clipped close. If the sample is a scraping, perhaps they can take it from the cuticle or close to the cuticle.
If you're being tested regularly as part of the custody case, you should familiarize yourself with the tests. If you have an attorney and can afford an expert to refute the testing method, that would be ideal. It really depends on what they're testing for -- recent use or previous use.
For recent use, the fingernail test is probably inappropriate. See the link in Resource(s), below. It's a PDF file and you'll need the free Acrobat Reader software to read the file. If you don't have the Acrobat Reader, you can download it from www.adobe.com.
I couldn't find any information about false positives being caused by food, drink, or other ingested items. It could be lab contamination -- even reputable labs can miss on minute contamination. Is it possible to get a statement from the lab on their standard contamination levels for THC? How do they eliminate for cross-contamination of samples? The test result was positive but was it a strong or weak positive?
It could be slow metabolising of THC. Note that THC metabolises very slowly and can take 25-30 days to be clear. So if you stopped 10 months ago, THC may still have existed in your system 9 months ago. Theoretically, you should not be testing positive at this point, but it's possible.
Good luck with your next tests and best wishes for turning your life around and staying clean.