I would definitely have this checked out by your primary care doctor
at your earliest convenience. There are several possible causes for this lump, and they will be able to take a close look and help you determine whether you need any additional treatment.
Make sure to mention to your doctor if you remember any trauma to the area, such as bumping your leg against a hard surface, because by far the most common cause of these types of bumps would be a bruise
or a contusion under the surface of the skin.
Another potential cause, depending on where exactly this is in relationship to your knee joint, is bursitis, which is an inflammation of the bursa, which are the protective sacs of fluid which surround and cushion the knee.
Depending on what your doctor sees, they may want to prescribe a medication to help reduce pain or inflammation. They also may recommend restricting your activity or at least avoiding undue strain on the leg or knee until this heals up. They will also be able to follow up with you to make sure that things are getting better and that you do not have any lingering symptoms.