The Blues, promoted via the play-offs from League Two last season, went three points clear of Swansea as they set a club record of eight wins on the trot in the week that manager Steve Tilson was named League One manager of the month for September.
Both teams started with all guns blazing but it was Forest who nearly drew first blood when Nicky Southall's half-volley from 25 yards forced a fine diving save from Darryl Flahavan in the Southend goal.
The visitors maintained the pressure with James Perch heading well over from Kris Commons' cross, before Jack Lester repeated the trick on 15 minutes from strike partner John Thompson's flick on.
Southend struggled to make much impression in the final third of the field until Mark Bentley rose to meet Kevin Maher's free-kick on 21 minutes but on-loan West Brom keeper Russell Hoult saved the header.
Bentley turned provider with a cross that found veteran striker Shaun Goater eight yards out, but again Hoult was up to the task and gathered the five-goal hitman's header.
Luke Guttridge was named the supporters' club player of the month for September before kick-off and nearly rewarded the Southend fans with a goal with a 30-yard effort that beat Hoult but smacked against the foot of the left post on 31 minutes.
Thompson had the first shot of the second half on 47 minutes but Flahavan easily held his curling 30-yard attempt.
Yet Southend then raised their game as they had done in the 4-1 win against Yeovil a fortnight ago. They started piling on the pressure around the visitors' 18-yard box and saw skipper Kevin Maher blast two attempts wide.
Forest manager Gary Megson introduced Eugen Bopp and Gareth Taylor in an attempt to stem the tide, but saw Bentley headed just over the crossbar from Maher's cross.
The Forest goal enjoyed a charmed life as the hosts failed to capitalise on a succession of corners, before Guttridge set up Goater only for the Bermudan to fire across goal and narrowly wide.
Southend boss Steve Tilson made a double substitution to bring on Eastwood and Jamal Campbell-Ryce, and after just eight minutes on the pitch Blues' top scorer snatched the winner.
Eastwood fired an Andy Edwards pass wide of the post on 79 minutes but found the back of the net three minutes later when he slammed home Bentley's headed cross.