Personally I wouldn't think that much of how she acted in "A Bird in the Hoof" (all the characters except for Fluttershy and Twilight pretty much just made an appearance to be silly or humorous, like aside from pinkie, rarity).
Moving onto this episode I don't think her wanting to throw Gummy another party right after was necessarily creepy, just more along the lines of silly.
Her entire flip out though I see as insecurity and I find that even more plausible by both how readily she jumped to the conclusion that her friends don't like her parties = not liking her... and her stalking of them. Actually the stalking and nosiness isn't so weird because I've known women that act like that, they get jealous or suspicious or just need to pry their nose if they feel left out of something.
But it seems that, despite being outgoing, she's really not good with people/other ponies. Parties are her thing, that's what she knew. She doesn't have much else to give people... or even worse she doesn't REALIZE she has much else to give people. If you're doing something that has brought you happiness and friends and you've convinced yourself that's essential to why they're friends with you, the thought of them not liking whatever it is you're doing in turn would mean they don't like you as well.
And... then she snapped. Why? Probably because it was funnier to write and animate then having a sad pony mope about for 5-6 minutes on screen. But if you want to take that as a character trait it's because she became bitter. She can't see what else she has to give her friends (if she does, anything) and she only thinks people like her for one thing. So she resorts to trying to rebuild and reinforce that one aspect of herself she does know people like about her. There's still a ton of ugliness in that but I could find reasoning to understand and sympathize with her situation to an extent.
I just read way too much into a pink pony flipping out because she forgot her own birthday.