Found a swollen immobile lump roughly 1.5cm in length at the rear junction of my buttocks and thigh. what could it possibly be?(i'm 24yrs old)

Possible abscess: would be concerned that it is an abscess, particularly if it is red or painful. if it is an abscess it will require drainage. it could also be a cyst, or a benign fatty tumor. far less likely it could be some sort of malignancy.... given the wide variety of possible diagnoses, it is important that you be seen by a doctor, to make a definitive diagnosis

Epidermoid cyst: This benign cyst, aka epidermal inclusion cyst, is the most common skin cyst. It typically occurs in areas of little/no hair, such as you describe, though it originates from hair follicle. It is usually filled with malodorous "cheese like" material. Rarely they become inflamed/infected. They don't require treatment; if inflamed a doctor may inject trimancinolone (a steroid) or surgically remove