Advice please please please.

One of my almost five month old twins feeding has gone to the dogs. During the day, she will hardly eat anything, last feed of the night she will feed for well over an hour and during the night, feed for about two hours. This is something that has only started in about the last three weeks and I have no idea how to get her feeding properly during the day again.

During this night, she fed from 1.25 to 2.30 and woke again at 3.45. I gave in to her grizzling at 4am - it's now 5am and I just tried to take her off, thinking she was just comfort sucking and she immediately latched herself back on.

I'm sure it isn't a supply issue, I've never had problems with supply (long may that last) and my other boob is very leaky as she is feeding now.
Help!!!