A pump is a temporary state in which the muscle is full of blood. Lactic acid is formed when glucose is metabolized during anaerobic exercise.
The lactate system in the body expends energy to synthesise ATP without oxygen - this is anaerobic glycosis. The result of Glycosis are excess formation of hydrogen ions andh pyruvic acid. Hydrogen ions formed too quickly (if they are formed to rapidly for the body, the hydrogen ions can't be shuttled away and turned into h2o by combining the H+ with oxygen) irritate nerve endings and inhibit the muscles normal function.
It is not the lactate that irritates the muscle, it is the hydrogen ions that change the pH of the muscle cell. Also, the pump is totally useless, there is not a single study that indicates otherwise.