hi i had gone through your query and understand your concerns. i would come up with,the most possibilities to fit your condition .these include
1.the first one may be, a CYST
and FNAC may confirm the diagnosis.
2.second possibility is,ARTHRITIS
of the acromian-clavicular joint may produce similar condition.
x-ray or MRI scanning may confirm the diagnosis
there is an another possibility like LIPOMA;is an fatty tumor
and not harmful
consult your orthopedist for correct diagnosis and treatment option.
I hope this is helpful for you, thank you