The thing is, nothing has changed in my fit for at least a few years. So while I’m not ruling fit out, it must have been triggered by something else?
Maybe the issue is your working your hamstrings too much in weight training. Maybe you fatigue them by trying to strengthen them and you aren’t able to recover.
I have only started back on weights in the last month, this issue started back in April and I was doing very little weight wise then.
The only other input I have is that it’s possible you may have an injury in there that you’re chronically aggregating and not allowing it to heal. It may not present as pain until they get tired.