Welcome to health care magic.
1.There are two possibilities . 1.
Hernia ( which is seen as increased and reduced in size as you mentioned) 2.
Hydrocele ( collection of fluid).
2.Other possibilities of cancerous can not be completely ruled out - but cancerous / mass lesions do not change there size as you mentioned.
3.So solution and investigation of choice in this case will be
ultrasound scrotum including
inguinal canal - will give exact condition.
4.That will give the nature of the swelling, its source and extensions.
5.Mean while the supporting under wear will be good enough, do not lift weights. Does not seems like a cancerous issue. Let it get examined and treated accordingly.
Good luck.
Hope i have answered your query,
any thing to ask ? do not hesitate to ask.
http://doctor.healthcaremagic.com/doctors/dr-ganesh/62888