lump at vaccination site

[HunterImagery]

Hello,
2 of my 3 boys developed lumps just smaller than a ping pong ball where I gave them their annuals. The 3rd one just lost a little hair.
My vet didn't seem to be too concerned and said they would probably go away or I could aspirate them myself. The lumps seem to have taken about 3 weeks to be noticeable and have not gotten bigger. The boys are all in good spirit and are not sick in any way. Any thoughts?
Threw up some pics of my boys, Schmidt, Winston and Coach. 15 months old and growing like the weeds they eat. Especially Coach, he is a brute and like his brothers is in great mountain shape.
 

That happened to my three boys last fall after I gave them tetanus shots. It scared me as well but my vet wasn't concerned and after a few weeks the bumps went away and haven't come back. The bumps were about the size you described and had no hair on them.   When we lanced and drained them they were filled with pus like a giant zit.