Why does Juliet think the Friar might have given her poison?

Juliet questions whether the friar might have given her a poison "lest in this marriage he should be dishonour'd / Because he married me before to Romeo." She is saying that she is afraid the friar might have poisoned her in order to protect his own reputation and, indeed, possibly his soul in the eyes of God. That is, she is concerned that the friar, like Juliet herself, is afraid the planned marriage to Paris will have to go ahead, despite the fact that Romeo and Juliet are already married. As such, if Juliet were then to marry Paris, in addition to her current marriage, the marriage would be bigamous, which would cast dishonor upon the friar.
This is only one of the concerns that flash through Juliet's mind at this point. She is also worried that the mixture will not work at all, in which case, still, she would be forced into the planned bigamous marriage. Juliet seems to feel at this juncture that she can trust nobody but herself. She is taking a great risk in trusting that the plan that has been concocted will actually work, especially because it relies on so many variables coming together.