My girlfriend has been diagnosed for thrush by the doctor a couple of days ago, but she is still in agony. Could it be a mis diagnose by the doctor? Could she have an std?

We couldn't possibly answer this question without a physical examination. I suppose just about anything is possible.

Has she called the Doctor who diagnosed her?

She has a burning sensation when peeing which makes it very awkward, also when showering she is in discomfort, and also when walking and bending. So the vagina is very sore also? Is this just extreme thrush?

Only a Physician who has examined her can diagnose the problem - and, yes, it could be an STD or just about anything else.

As your other answer said, these are questions for a doctor. Please ask her to go to a different doctor, if she feels this one hasn't helped her. Good luck.