As an eligible employee, you are allowed 12 weeks of FMLA leave each year. The method by which the "year" for FMLA purposes is determined varies employer to employer. The employer can choose to provide FMLA leave calendar year to calendar year (e.g., Jan. 1 to December 31). Other employers choose to provide FMLA on a "rolling" basis, which means that employees are eligible for another 12 weeks of leave 1 year after the last date on which the employee first took FMLA leave (e.g., if you took all 12 weeks of FMLA leave in 2011 beginning on July 1, 2011, you would not be eligible for another 12 weeks of FMLA leave until July 1, 2012).
So, if your employer works on a "calendar year" basis, and you took 12 weeks of FMLA leave for your Achilles heel in 2011, then you may have become eligible for an additional 12 weeks beginning on Jan. 1, 2012. If your employer uses the "rolling" method, then it is unlikely that you have any more FMLA time in which to use. You should talk to your employer and see whether you have any FMLA time remaining for 2012.
If your employer has never made a specific designation as to how it calculates FMLA leave, or did not effectively communicate that designation to its employees, you may be entitled to take advantage of a method of calculation that is most beneficial to you.
Mr. Davey has provided you a good overview of the Family and Medical Leave Act.
What's missing from your question (and the reason neither he nor I can specifically respond) is that you haven't said how much of your 12 weeks you used up due to the torn achilles, and over what period. If, like Mr. Davey suggests, you used up 12 weeks for the achilles injury within the last year, *no*, you cannot take any additional leave this year. If you used up less than 12 weeks for the achilles injury, you can take up to an AGGREGATE of 12 weeks for the stress fracture.
You don't get an "automatic" grant of FMLA for the stress fracture just because you got FMLA for the achilles injury, and you don't get 12 weeks of leave "per injury," only "per year."