I have a, what i think is a small pimple, under my arm. but it has two white heads on it, and it's painful. is it a pimple or something else?

Impacted follicles.: Sounds like this would be two impacted underarm hair follicles. This can occur due to compression from clothing, use of underarm deodorants and anti-perspirants or friction from shaving in the armpits. If you develop fevers/chills/sweats, or if the area worsens acutely, you should be seen by a doctor as this could represent something more in that context.