There are many things that can cause frequency, including over active bladder, chronic cystitis or interstitial cystitis. A good urologist can help diagnosis the problems.
We have an IC/ Pelvic pain support forum here on this site actually two, listed below. I am also an IC support group leader and director of MOARK IC. If you need more info or some links feel free to pm me. There is even a link to a questionnaire that can
to tell you if it is likely that you have IC, this is a diagnostic tool used by doctors so you can take the results to your doctor and discuss it with them.
You also might want to consider a referral to a pelvic pain PT. They are often able to rule out and pelvic issues, muscular etc. that doctor's are not trained to look for.
http://www.medhelp.org/forums/Interstitial-Cystitis-IC--Pelvic-Pain-Support-Forum/show/412
The expert forum where you can actually ask the doctor here on Medhelp is here also:
http://www.medhelp.org/forums/Interstitial-Cystitis-/show/264
Hope this helps!
Shelly