This is recurrent herpes labialis caused by a reactivation of the herpes simplex virus which lies dormant in the trigeminal ganglion until something triggers it to proliferate. It seeks out peripheral epithelium and forms vesicles that eventually break open. Here, multiple vesicles have coalesced to form a blister or bulla. The sun, fever, trauma, emotions are all triggering factors.
Healing involves a crusting stage and can take weeks.