Welcome to Health care magic.
1.The history given by you suggest me- the most possible cause could be
ganglion cystic structure arising from tendon, bur-sae,
nerve sheet.
2.They are generally pain less lumps adjacent to joint space.
3.Your further step should be get an
ultrasound lesion / lump done.
4.Ultrasound will evaluate the source of the lesion, its content and where it is extending.
5.But best investigation of choice is
MRI joint - good tool for evaluation of nerve, tendon, bur-sae, nerve. and content of the lump. Good luck.
Hope it helps you. Wish you a good health.
Anything to ask ? do not hesitate. Thank you.