I think its more likely to do with the end of Scarlet Spider #6. Julia said that "She is coming back for Kaine." My guess is that its something to do with that. Probably the Queen.
Julia was already tied up with the Superior problem when this happened she probably didn't have time to look out for both Pete and Kaine. So she had to do the best she could for both. Not to mention she would be the first to know if the Queen was back. We saw her have the same migraine things during Spider-Island. My guess is that the letter is warning him about the Queen coming back for revenge after Kaine killed her.