DOMS (delayed onset muscle soreness) is the soreness that sets in with a delay in muscles you have loaded in an unfamiliar or hard way. It is a sign that your body is repairing stressed muscle fibers.
The more unfamiliar the load, the stronger the response. Your first session after a break, a new program or a clear jump in volume causes soreness more readily than the tenth identical workout.
DOMS and body weight
During repair, fluid collects in the stressed tissue. Together with refilled glycogen, this can lift the scale for a few days. It is neither fat nor sudden muscle mass.
Example: After a hard leg day your daily reading jumps from 82.4 to 83.2 kg, while your trend weight rises by only about 0.1 kg. A few days later the scale shows 82.3 kg again. To gain 0.8 kg of fat you would have had to eat roughly 6,200 kcal above your needs. This is a worked example.
So when you start a new program, give the trend one to two weeks before you adjust calories. A marker on the start date helps you attribute a water step correctly later.
When it is too much
Soreness is not a measure of a good workout. If it has not faded by the next session for the same muscle and your performance also drops over several sessions, less volume or a lighter week is usually the better choice. Persistent pain, swelling or injuries are not a training stimulus; get them checked by a doctor.