We'll find out when she decides we are to find out. The Palace doesn't leak.
The BBC has nothing on it, which is itself an indication of something larger than normal.
It could be something fairly innocuous - the Order of the Garter ceremony on 19th June has had to be cancelled so Herself can swear in (or preferably at) the new parliament. That's something she's not going to be liking, she's never missed one, ever.