I've had a bump on the underside of my penis that is similar to the size of a mosquito bite for a few days now. It is only painful when touched but I don't know what it is.

Neither do I know precisely and surely what it is . So see your physician asap. He/she should know and be able to treat it !

Success!

I agree with the previous post. Just get to a doctor as soon as you can. It could be anything from an insect bite to an allergic reaction or more. The "more" depends on how sexually active and/or indiscriminate you might have recently been. Whatever it is don't worry about it as almost all these things are easily treated. But you really must see a doctor about it and I would suggest, within the next few days.