John Gill Commentary 2 Chronicles 21:15

"and thou shalt have great sickness by disease of thy bowels, until thy bowels fall out by reason of the sickness, day by day." — 2 Chronicles 21:15 (ASV)

And you shall have great sickness by disease of your bowels ,
&c.] Which was a just retaliation to him, for having no bowels of compassion on his brethren whom he slew:

until your bowels fall out by reason of the sickness day by day ;
that continuing day after day, and even year after year, the space of two years, as appears from (2 Chronicles 21:19) .