M.,
I could have written your question. I am in the EXACT same place with my 4 1/2 month old son. He was a great sleeper at first, ate great during the day, and slept an 8 hour stretch at night. Now, he is up all the time, and will go LONG stretches during the day. We were on an outing this past Friday morning (at an animal farm with my 2 year old) and my baby wouldn't eat for the whole 6 hours we were out of the house. I kept trying to get him to eat (mostly because I was getting uncomfortable and was worried about leaking all over) but he was just way more interested in what was going on around us to take time to eat. Needless to say, he was up all that night eating. He also doesn't do a bottle (glad to hear I'm not the only one with that problem too!).
I do notice that if my 2 year old is out of the room, or if I go into our room and use the rocking chair and boppy where I feed him at night, he does much better (I don't use a boppy usually during the day). My older son just turned two last month and is still in a crib, so sometimes I will put him in there with a pile of books and cars and then feed Joey, if there are less distractions it really helps him finish his meal.
I have also used (a few times) a necklace with big beads on it to keep him focused on me during nursing, and that seems to help.
Good luck, and I hope to see some other advice as well!
J.
SAHM to 2 year old Charlie and 4 1/2 month old Joey.