lip pimple

Recently every so often I would get this itchy,swollen pimple sized red bump on the border of my under lip and the skin.

It occured three times already on the same spot, maybe once a month type of thing. it hurts when I touch it and also itches..
All three times when I popped it and there was white stuff popped out like when I pop pimples on my face.
After it was popped, it healed right away.
To me, it looked different from cold sore because when I do get cold sore, it has blister all over the area and
later on the colour changes to yellowish green.

What could this be? Is this just infected pimple or something I should be concerned about?
Could this be STDs?

[Dr_Aparna]

Hi,
This could be  a pimple occuring there because of excess cream/lotion at that point.Acne is a skin disease caused by changes in the pilosebaceous units (skin structures consisting of a hair follicle and its associated sebaceous gland).
Acne develops as a result of blockages in follicles.Widely available OTC bactericidal products containing benzoyl peroxide may be used in mild to moderate acne. The gel or cream containing benzoyl peroxide is rubbed, twice daily, into the pores over the affected region.
Externally applied antibiotics such as erythromycin, clindamycin can also be effective in eliminating the bacteria at the site.
ref:http://en.wikipedia.org/wiki/Acne