Sounds like a frayed line, we've all done it fishing heavy cover and not retying and cutting off five or six feet of line occasionally.

I have used the Shaw Grisby knot for years on every kind of line no problems check for nicks look at your line guides

This. If you have even the smallest tag hanging out and make a cast, it will fray the line at the tag. Take the time to get those tags out before every cast. Unless you're one of those guys that never backlashes.

I use this same knot on everything except crankbaits with no problems...I mainly fish jigs with 18-20lb flouro if I'm in open water though.

I have always used the San Diego Jam Knot. I thought it was called the Fox Knot. That's what John Fox the American angler called it any way. That is where I learned to tie it. Do not use it on Braided line, learned that the hard way. Then I learned how to tie the Spider Knot or other wise Known as the Palomar Knot.

i had problems also with invizix 15 flourocarbon and a palomar knot( even using the youtube video how to tie a palomar with fluorocarbon)

i changed to Sunline Sniper and problems when away with same knot....

I always tie a Polymar with Invisx. I now use chapstick when sinching down the know. No issues.

Good ol fashioned clinch knot.....never had any issues with fluro or mono...40 years of fishing

I changed line went back to same spot caught a 6lbr no breaks. Guess line was bad or knicked. Silly it would be fine with 3s but a big fish instant snap.