AlwaysI had something similar happen once - Settings told me my available iCloud storage was something like -12GB. Are you on a beta by chance?
This gives me some idea. Curious about how long it's going to take me to read my 2.4 million active unread emails.If you read one million messages per second (1 microsecond/msg), it would take about 292 years to get through that many. Or in more biblical terms, "unto the 10th generation".
Oh, and that's assuming the servers stayed up that long.
That number is 2^63 - probably a 64-bit signed integer that rolled over the wrong way and got presented as an unsigned integer (my code does that all the time!).