Chazal (gemarra megillah) tell us that there is a difference between the words ‘Vayomer’ and ‘Vayedaber,’ even though both seem to mean ‘and he said.’ Vayomer denotes speaking with calmness/kindliness, whilst Vayedaber means speaking with harshness/a firm expression.
In our sedra, when Pharoah first asks Yosef to interpret his dream, the Torah uses the Vayomer term, but after Yosef tells him that all his dream-interpreting powers come from HaShem, Pharoah moves to the Vayedaber term in telling over his dream. Why the switch? Perhaps one can suggest that Pharoah did not want to hear about HaShem’s Powers at this point – he wanted his dream interpreted and nothing else.
Therefore, he was slightly angered by Yosef’s response of piety and so then spoke harshly; hence the use of the Vayedaber term.