It's good that your doctor has actually referred you elsewhere for further checking. Some doctors don't do that and if anything is going on in that area it ends up being missed. This has happened to me and i'm in the middle of sorting out something which has likely been going on for years and nobody has bothered to look into things further.
I hope they do a biopsy to cover all bases so you know precisely where you stand. It could be to do with anything ranging from minor things like yeast, but i've not heard of white spots and yeast, usually they might say there is unusual amount of discharge or something like that.
I have read of people having white spots on the cervix relating to herpes and genital warts. Sometimes these types of things have no symptoms and could be there for a while before they are picked up by a pap smear. Herpes will come and go, but from what i've read warts stay. If you have had a full STD check up, including herpes and it's all come back negative then i guess it wont be an STD, although they do say we all pick up some form of HPV strain when we have sex with someone. Most often the HPV will clear up all on it's own, in other cases antibiotics might be required. Certain strain/s of HPV are responsible for genital warts, but not all and i think it's mostly the number 16 strain of HPV which causes cervical cancer, not sure if the other strains are also responsible.
Lastly, the only other thing connected to white spots which i've come across in reading about gyno health issues is cervical cancer. This form of cancer is said to move slowly, but regular pap smears are encouraged so they can quickly do something about it if it is found.
I hope all goes well at your next appointment and i hope i've not scared you too much, but it's always an anxious time when dealing with any women's issues below the waste as it can be something serious, but most times it ends up being something minor. The main thing is that you have a good doctor who doesn't dismiss things as simply nothing and has taken the correct precaution in referring you on for a full check up.
Best wishes and let us know how you go, take care