According to insiders, Rob is 'super angry' that Kristen has been staying at the home of her producer pal, and thinks there could be something else going on.
Sources say Kristen Stewart has been hiding out at Agnelli's plush Hollywood home ever since news of her affair with Rupert Sanders broke.
But now Robert Pattinson's on the warpath.
'Rob knows Agnelli and has been calling him a lot since Kristen has been there,' one insider told Radar Online.
'He wants to know why Kristen is staying there at his house?' the source added. 'He is super angry now and has accused Giovanni of sleeping with Kristen too.'
Giovanni Agnelli first hit the headlines last week, when he unleashed a Twitter tirade defending Kristen Stewart from critics.
'I'll say right now what the manipulative media isn't saying... Kristen Stewart did not have any type of sex with Rupert Sanders,' Agnelli wrote.
'The spin that keeps growing on this story is staggering,' he continued. 'There was NO on set affair. Stop printing LIES.'
'If you've ever used the word Trampire you're a mindless pathetic piece of s***.' he added.
Meanwhile, insiders say Robert Pattinson and Kristen Stewart are talking again, after Kristen bombarded Rob with 'tearful texts'.
But it's thought R-Patz still has a long way to go before he's ready to forgive his cheating girlfriend.
Do YOU think Kristen Stewart's had a second affair? Or is Robert Pattinson jumping to conclusions? Will Rob and Kristen ever reunite? Let us know in the comments box below…