was not having a good day. On July 26 the actress was briefly hospitalized (her camp blames dehydration and exhaustion) and subsequently missed work on her upcoming drama Georgia Rule
. Then she received a scathing letter from her boss, Morgan Creek Productions head James G. Robinson, who called Lohan's on-set behavior "unprofessional." He also questioned her illness, writing, "We are well aware that your partying is the real reason for your so-called exhaustion." So how did Lohan, 20, respond to her dressing-down? By hitting the town the next five nights—mostly with beau Harry Morton. On July 29 the couple even jetted to Vegas, where they caught the comedy show Beacher's Madhouse
and, says an onlooker, "They were the life of the party." Just hours before falling ill, Lohan was also seen living it up at Guy's bar in L.A. on karaoke night. But Lohan's mom, Dina, disputes that partying sent her daughter to the hospital. "It was 105 degrees on the set and she has bronchial asthma," Dina told PEOPLE. "She was weak from the heat." A Lohan pal concurs: "Honestly, she hasn't been crazy partying. In fact I've never seen her so together." Either way, the day after her absence, "Lindsay was on-set for her 8:30 a.m. call time," says the pal. "She said she had a great day at work."