welcome to health care magic..
regarding your query,
common swellings in the scalp
are sebaceous cyst,,it is round mobile painless unless it gets infected..
you told that it is irregular light brown color that too changed recently
mam don be panic,,it looks like an melanoma which may be malignant...
CHANGE IN THE COLOR OF EXISTING MOLE IS AN WARNING SIGN OF CANCER..
immediately with out getting delay consult dermatologist
or general surgeon
to rule out malignant melanoma
..he will tell you the exact diagnosis and the mode of treatment..
hope you understand my suggestion
have a speed recovery..