What's wrong with my cat?

[bobman0330]

For the past two weeks or so, my cat has been spending maybe a third of her waking time wandering around my apartment, meowing pathetically constantly. Obviously I'd like to figure out what the problem is, so if anyone has any suggestions, I'd be very much obliged. Here's what I know:
-The cat is a little over one year old, female, fixed.
-She'll meow even when she has food, water, and a clean litter box.
-When she's in a meowing mood, she's not interested in affection or play, although she'll sometimes be distracted if I vigorously shake her favorite lure.
-Sometimes she'll fixate on the main door of my apartment or the door to the closet where her food is kept. She will scratch at the door or sometimes leap at it. She keeps doing this even though I spray her with a squirt gun almost every time she does.
-I thought she might just be lonely, so I took her to my gf's apartment. She has an older female cat (who strongly dislikes my cat FWIW). She tries to play with gf's cat at about the same frequency as she has in the past, but she's still meowing a lot.
-I bought her some new toys and a harness/leash so I could let her explore a bit in the halls and outside. She seems to like that OK, but again, no end to the meowing.
-She almost never does this when I'm asleep, although she frequently will if I'm in bed reading.

Again, if anyone has any advice, it would be very helpful. I'm starting to worry that my cat is suicidally depressed, plus the meowing seriously impairs my ability to get stuff done at home.