I apologize for my delay in getting back to you. I live way up in Illinois, so I have had to do a little research on St. Augustine grass. It does not grow up here.
Yes, your thought that your grass has a herbicide problem could be correct. According to information from Oklahoma State, Mississippi State, Alabama, & Univ. of Georgia, the phenoxy herbicides need to be used very carefully on St. Aug grass. This class of herbicides is used very commonly to control broad-leafed weeds (i.e. - not grassy plants). The herbicides in this class include: 2,4-D; dicamba & MCPP. According to the literature, these herbicides can discolor and weaken a St. Augustine grass stand. (http://www.cropsoil.uga.edu/weedsci/slides/agt01/agt01.ppt) Another herbicide commonly used to control grassy weeds in turf is the organic arsenicals. This class of herbicides will seriously damage St. Augustine. Herbicides in this class include: CMA, DSMA & MSMA. (http://www.okrangelandswest.okstate.edu/pdfFiles/OSUextPubs/F-6421.pdf)
How can you determine if any of these products were used on your grass? Check the bill from the "Lawn care" company - it is supposed to show all products that were used on your lawn and their application rate. Testing laboratories can detect trace amounts of herbicides in the soil, but these are usually expensive tests. A & L Laboratories can do an analysis for the phenoxy class. (http://www.al-labs.com/) - $250/sample
Have you contacted your local extension office in Jacksonville? They are located at: 1010 N McDuff Ave., Jacksonville, FL 32254-2083 (904) 387-8850 http://duval.ifas.ufl.edu/
Hope this is helpful-