go see a doctor
Have you noticed any discolouration? Redness, bruising or swelling in the area you feel the pain? Where exactly are you feeling the pain?
There is a ligament that tends to hold the penis upright. This can be injured or even snapped if the accounts I've read on this forum are true.
Pain is a warning something is wrong matey. Gillette has a good point, is there any visable damage?
You just woke up one day with a sore dick?
Well, do you? Experience any pain if you apply pressure to certain areas?
If the pain has been consistent or worsening seek medical counsel sooner rather than later.
See a doctor, the last thing you want is an untreated burst blood vessel turning into a internal haematoma on you.