What are the causes of a red pimple on my eyebrow?

You need to be more specific in describing the lesion on your eyebrow. STD's do not manifest themselves as a pimple on the eyebrow. Although, if your pimple does not go away, it could be an abcess (collection of pus secondary to infection). If it is an abcess, then you could apply topical antibiotics, or simply pop it and drain the pus.