The cause of the swelling is more than likely from being Castrated. There may be some swelling above the band, but not a significant amount. This is generally from the fatty tissue in the scrotum and may sometimes rollover the band. If the swelling continues above the band, consult your veterinarian. If swelling is below the band, this is a result of the band not being administered tight enough.
Re-administer another band immediately, if you are able too, over the existing band. “DO NOT” remove the existing band, leave it in place. If you are not able to put another band on, find an old inner tube. Cut a strip about 1″ wide by 12″ long. Wrap around the existing band as “Tight” as you can possibly get it several times then tie off the tubing. You may also use surgical tubing, but it is difficult to tie off with out the knot slipping.
The ONLY reason a band isn’t tight is from the band not being pulled off of the spreader bars and/or the tightening rod not being pulled back as far as it will go. This happens quit often with first time users or users who castrate a small number of livestock once per year and forget this step. A sign of the band not being pulled off of the spreader bars is the spreader bars will be bent inwards. If you feel comfortable, you may cut the scrotum off, BELOW THE BAND, then apply another band.