If you have knots that are near permanent, you need to do a refresher on sterile injection procedure. Aside from that, make sure your gear is just carrier oils, BA, and if it has to have any barely enough BB to do the job.
If you inject contaminated substances, your body fights the bacteria by isolating it in a cyst then attacking. Most of the time people use contaminated gear, they don't get a systemic infection because their immune system can handle it fine, but you end up with near permanent knots from the cyst pockets left behind.
Make sure you are alcohol/Chlorohexadine padding your skin and vial, make sure you're clean in the first place (hands washed, showered recently etc) draw with a different needle than you shoot with, etc.
There's other reactions that can cause it too, intolerances to EO/BB etc, but minor contamination/localized infection are major culprits.
My only couple permanent knots are places I got slight cellulitis from contaminated material.