Hi, Dr. Hurst here.
It may indeed be a pimple, just like people get, only we call them pustules. A pimple or pustule is simply a small infection in the skin. It is usually caused by bacteria that normally live on the skin. Something can irritate the skin and allow the bacteria to enter and cause an infection. Sometimes a pore or hair follicle can get stopped up with oily secretions or sebum and a pustule will result. Sometimes it can be secondary to trauma from her scratching or clippers if she is groomed. Other times it can indicate an underlying allergy that causes irritation and allows the bacteria to get a foothold. If she has an allergy we would expect her to be itchy. If there is only one or even a couple, often a topical antibiotic cream applied 3-4 times daily will be enough to treat the infection. If there are lots of areas of the skin affected, which you may find if you start looking through her fur, an oral antibiotic may be necessary.
If it is not a pustule, it could be an insect bite. If you live in an area with red ants, their bite will cause a lesion that looks just like a pustule. In that case, there is no infection, just a reaction to something in the ants saliva. In this case, an antihistamine or steroid cream may speed along the healing and minimize itchiness.
Sometimes dogs can get little skin tumors that appear white as well. These do not have a white "head" like pustules do.
So, if there is only the one or very few pustules, I would try the antibiotic cream first. A nice cool bath with an antibacterial dog shampoo wouldn't be a bad idea either. This will help wash away bacteria from the skin as well as oils and dirt that accumulates on the skin.
If the lesions do not resolve or if you find many lesions or if she seems to be getting more, she may need to go to the vet for an exam and possibly some oral antibiotics.
I hope this helps.