Hmm - well, the fact that it doesn't go away after 2 weeks, and you've had this before in the same spot, leads me to believe its not a pimple. My guess is its a Herpes Simplex virus, an STD. The virus will manifest itself from time to time in the form of red, pimple-like blisters on the genitals (basically a cold sore). They can go away, then come back from time to time.
You should really have a doctor take a look and diagnose this though. If you do have Herpes, its not a big deal - a lot of people have it, and it usually doesn't do anything more than simply cause these red bumps/blisters from time to time. But if you do have it, you should be aware of it, as you can spread it to your sexual partners, particularly when those red bumps appear.
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red lump on the shaft of penis 
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