What causes bony lump at the end of clavicle? It is definitely a part of the bone, not a lymph node

I understand your concerns. It's natural to worry about unusual or asymmetric features on our bodies, especially if they've just recently been noticed. I'll try to provide some perspective:

Normal Asymmetry: The human body is rarely perfectly symmetrical. Many people have minor differences from one side of the body to the other, whether it's the size and shape of hands, feet, ears, or, in your case, the clavicle.
Bone Variations: It's entirely possible that what you're noticing is just a natural variation in your bone structure. As you correctly pointed out, the sternal end of the clavicle is where it articulates with the manubrium of the sternum. There's a joint here, the sternoclavicular joint, and there are many ligaments and tissues in this area which can also contribute to the overall shape and feel.
Bone Conditions: While tumors in the clavicle are rare, especially primary bone tumors, it's worth noting that many benign conditions can cause bone prominences or changes in the shape/feel of bones. For example, osteochondromas are benign bony growths that often appear during growth and can be palpated as hard, bony prominences.
Trauma or Past Injuries: Sometimes, small fractures or injuries from the past (even if they weren't severe enough to be noticed at the time) can result in minor changes to the bone's shape as it heals.
Anxiety & Health Concerns: Health anxiety is a real and challenging issue for many. Once we notice something unusual about our bodies, it's easy to fixate on it and jump to worst-case scenarios, even if they're statistically unlikely.