Bell told club officials and the players of his departure after the result, said team manager Gareth Williams, who praised his close friend and clubmate.
A statement from the club explained that Bell had left due to personal reasons, having discussed his position just last month.
Bell explained his concern over balancing his new job and spending time with his young family alongside his role at Regentsholme.
“Duncan and the club had discussed his position in mid-October; he felt it was becoming more difficult with increased family and work commitments,” the statement read.
“He was of the opinion that if he could not give Lydney RFC 100 per cent commitment he was not prepared to carry on.
“The club persuaded him to give it until Christmas and think again. It however became increasingly more difficult for him, so he took the decision to step down on Saturday.
“He is an excellent coach and will take up a similar position and be successful in the future. Lydney RFC would like to take this opportunity to thank Duncan for all he has done wish him every success in whatever he chooses to do.”
“There is never a good time to change jobs,” Williams said.
“From our point of view now we have got to keep the boys together. We want to get someone in to coach the forwards and we will be looking at a replacement but we don’t want to have a knee-jerk reaction.
“We still have a game on the weekend in Brixham and Chris Holder and Sam Arnott, who are both disciples of Duncan, will stay on in the coaching staff.
“I think Duncan’s a good coach; he’s a good mate and he want someone who is going to continue the good work he has done.
“We aren’t far off the top of the league and it’s not the right time to get someone in and just restart. The new director of rugby will need to be similar.”
Lydney’s win came after the Severnsiders went 13-5 down at half-time following tries by Ivybridge’s Billy Pinkus and Jon Morgan.
But Chris Rovery hit back with the first of his incredible four-try haul, while Tony Wicks added a penalty just after the break.
Four further tries were added as Rovery secured a hat-trick, while captain Sam Arnott also dotted down. Wicks and Joe Carolan added a conversion each.