I am a single filer, but my standard deduction on my federal return is 6690, despite the fact that it is supposed to be 13850. What could cause this to happen?

Are you a dependent?  

The Standard Deduction for an individual for whom an exemption can be claimed on another person's tax return is limited to the greater of:

…..$1,250 or

…..Your earned income for the year + $400 (but not more than 13,850))

So you would get a minimum of $1,250.

See Table 8 Standard Deduction worksheet for Dependents on Page 26

https://www.irs.gov/pub/irs-pdf/p501.pdf

Or the 1040 worksheet on page 34

https://www.irs.gov/pub/irs-pdf/i1040gi.pdf

@VolvoGirl Thank you very much. That helped me a lot.