I have a 4 cm long, 1 cm wide skin nodule in the upper left of my pelvis, that is mobile and tender since 1 week, with no other symptoms. what could it be?

Without seeing this, it would be difficult to determine what it is. It could be a benign growth, such as a lipoma, but these are often not tender. It could also be an infection/abscess which needs treatment. If there was trauma to the area, it could be a hematoma (bruise/collection of blood). There are also other causes...please see your doctor to be properly diagnosed and treated if needed.

Anything tender under the skin could be related to mild trauma or bump into a hard surface that can cause a small bleed under the skin. Watch the surface of the skin on top of this lump and see if it shows as a bruise. Most trauma related lumps will resolve and go away in 2 to 3 weeks. If it is still present or grows in size you should visit with a physician who can examine you and guide you furth