Are lactic acid and prolactin rise after exercise connected?
Prolactin (PRL) is a pleiotropic hormone. More than 300 different functions have been described for it (Bole-Feysot et al., 1998), although probably the most well known/studied1 is the maturation of the mammary gland during pregnancy and subsequent stimulation of milk production.

One of the fairly well known situations when PRL increases is after an acute stress (Neill, 1970; Seggie and Brown, 1975; Kant et al., 1983). Although the details of this are still a bit foggy, the reason seems to be connected to the anxiolytic properties of PRL (Torner et al., 2001, Donner et al., 2007).

Anaerobic excercise is considered as a stressor, and therefore the PRL response may be important to avoid hyperactivation of the stress pathway. This has been suggested before (Ohiwa et al., 2007) and the rise in PRL may be triggered by lactic acid, as administration of lactic acid alone can stimulate PRL secretion (Sculthe et al., 2013).
Note that this does not exclude that there may be other factors involved in this response. 



1 at least in female mammals, but bear in mind that PRL is present in all vertebrates, males and females alike.