This happened to me the other day ... took a lot of investigation, but in my case, turned out the X was continually trying to send an email with a photo attachment - a photo I had taken at 8 mp on my X, which was apparently too large for it to handle.
It was working on this for several hours by the time I noticed how HOT it was along the lower back part! Took the X out of its case to see if it cooled (not), looked for anything running (GPS, etc - not) and finally noticed the little green arrow by the email indicating it was still in process of sending... nearly 2 hours after I sent it!!!
Deleted the offending email and all is well. Wow.
PS - I later sent the very same photo via MMS where the X compressed the file. Went with no problems!