If it's not clearing up after several weeks of using an over the counter treatment, then yes, see your doctor. If you read the label and speak to the chemist, they'll both tell you the same.
Do you use any kind of intimate wash or deodorant - femfresh or some such? Someone once told me that using this stuff actually washes away the good bacteria as as well the bad, so they don't actually do you much good. I've noticed my cystitis tends to worsen when I use Femfresh, so I've stopped using it.
I did also hear that live yoghurt is a good treatment, but I've never known anyone who's used it, so I'm not sure whether you're supposed to eat it or smear it on/in, lol.