An itch, burn, or discharge is not always a yeast infection, regardless of whether or not the symptoms go away when you use an over-the-counter (OTC) product to treat a presumed yeast infection.
Many sexually transmitted diseases (STDs) have transient symptoms, causing an initial itch, burn, or discharge, but then those symptoms resolve on their own, although the infection is still there. This is not only true, but very common, especially with chlamydial infections.
The problem is that even though now silent, this infection can be damaging your reproductive tract, possibly even leading to infertility.
So, when is a yeast infection just a yeast infection (which is really a yeast overgrowth, not a true infection)? Well, if you are taking an antibiotic (which kills off the good bacteria in your vagina along with the problem bacteria causing an infection elsewhere, like your sinuses), and you have not ever had a sexual partner (or are in a long-standing monogamous relationship), it is likely yeast.
If you have had a new sexual partner, however, I'd suggest you go to your doctor and be diagnosed accurately.