It happens when Barnabas first begins to feed on Maggie. She becomes quite cranky and short-tempered with Sam, Joe, and Vicki, who are all merely looking out for Maggie's best interests. In one memorable scene from Episode 232, she instigates a big argument with Joe, throws him out of the cottage, and nearly breaks up with him. It was quite a personality reversal from the Maggie we usually see.
But that was only temporary while Maggie was a vampire victim of Barnabas Collins. She became cranky only while she was suffering the effects of Barnabas' vampire attack and spell over he. Once she was no longer a victim of Barnabas Collins, and her memory of those events erased by the "eminent Dr. Hoffman", she was back to being her amiable self, and in Episode 497 when she realized Willie had only come to her house that night to warn her of danger rather than harm her, she was as kind to him as to everyone else.
Remember also that when Carolyn was a vampire victim of Barnabas, she also became uncharacteristically callous (see Episode 355, where Carolyn responds rather coldly to the news of Burke's death and Vicki's loss, leaving Maggie appalled at her unusual behavior).
And when Joe Haskell becomes Angelique's vampire victim around Episode 560, he starts behaving bizarre and yelling at Maggie, at one point getting rough with her physically when she tries to stop him from leaving the cottage, after he had begged her to lock all the doors and not let him out of her sight.
