Welcome to health care magic.
1.There is a lump behind the are since birth but doctor said to monitor - probably clinically it was felt like
lipoma ( fat cell lump), or cystic structure which needs to be monitored.
2.If you are concerned with the lump then you need to get an
ultrasound scan of the lesion / lump to be done.
3.Ultrasound will help in analysing the nature of the lesion ( cystic , solid, mixed)
4.Its source ( from muscle, bone, vessels) and where are the lesion is extended.
5.If necessary an FNAC - fine needle aspiration cytology t identify the cells.
6.Depending on the findings, excision can be planned followed by antibiotic treatment. Good luck.
Hope i have answered your query.
Any thing to ask do not hesitate. Thank you.