Q: iPhone 4S Battery quickly drained and it got hot

I've had this iPhone 4s for about 18 months.  I had been working very well.  To keep it charged regularly, I put it in a Dock at my bedside every night and put it in a belt case (Sena Laterale Case for the iPhone 4/4s.  As I said, I've had no problems with it until today.  I very rarely find that I don't have enough charge for the entire day.  I use the phone more like a iPod Touch with cellualar data and occasional voice calls.  the phone has no permanent case or guard.  When it is not in the belt case, it is bare.

Phone:  4s with 32 GB memory

              OpSys:  iOS 6.1.3 - iTunes says it is up-to-date

             Siri - on but did not react as if I was talking to it. (it usually chimes when it hears me), raise to talk is on (I just turned it off as I find Siri nearly useless)

Today, when I got up, the phone seemed OK.  I used it a couple of times to look up something and I set an timer to remind me when it was time to do something. There were no telephone calls in or out.  There was one weather notification and 2-3 news alerts. After I got up, the phone spent most of the day on the coffee table.  When I got ready to go out, I put it in my belt case.  When I was waiting for my dinner, I got the phone out to look up something.  I noticed right away that it felt hot.  Epecially the back of the phone.  But the big thing was when I tried to wake it up (first the home button and when that didn't do anything, the on/off button on the top).  I coulld not wake the phone up.  I held the on/off button down for 5-10 seconds to try to reset it.  Nothing.  I gave up except for trying it a few more times.  After I got home, maybe an hour or hour-and-a-half later, I sat down and tried again.  Ready to consult the user guide for suggestions.  First I tried it with the home button and then  the on/off button.  Still no luck.  Phone still hot.  I then held the on/off button down for over 5 secs and THEN this time, a battery image showed up showing it drained with just a small amount of red marking the charge level.  I know what that means.  So I put it on a charger.  two-and-a-half hours later, it is at 90% charged.  It had been on the charging dock for at least 8 hours over night this morning.  I didn't check its charge level this morning but I often do and it says 100% if it has been on the charger all night.

Two things bother me about this:

a)  I'm sure I haven't used that much power.  The only time I've had the battery indicator go into the red is when I've been reading an e-book and I haven't done that lately.  I have a kindle for that or my iPad.  So it seems that my battery didn't hold as much charge as it usually does.  How can I find out if there is something wrong with it.

b)  The hot back is unusual.  Even when I've read e-books for an hour or two in the past it didn't get that hot.  I rarely watch videos of any sort on it and none today.

I may have to go to the Apple Store but as that is about 100 miles and two hours *each* way it uses up most of a day for a round trip including time spent at the store, I would rather see if there are other things I should try first.

Bruce