it might be a cyst which isnt anything to worry about unless it begins to hurt very painful and red... its just rather annoying is all.. if you want and if you can you can take a hot bath when it comes and rub it between two fingers to try and get it loosened and come out... the reason it comes and goes is because of hormone levels throughout the month... i hope this helps!
Have you spoken to your Gynachologist yet?
Is it actually a white head, or does it just seem lighter than the rest of the skin around it?
It could be a fat deposit, an area of skin that is weaker than the rest allowing protrusion, or perhaps a genital wart. Either way, go have it looked at...