Maybe you have a "spirited" baby. If you look it up on google, there are lots of "info." Just weed through it and see if it applies.http://www.google.com/search?q=raising+a+spirited+baby&am...
http://www.nurturingourfamilies.com/spirited/exptdefn.html
Every child is different, and maybe this does not apply to you, but My daughter was/is a spirited baby/child (not in every way, but in many ways)... among many wonderful things she was real clingy, and still is (but in a different way now), and although she was social...she was also noise sensitive (which your girl does not sound like) and she got overstimulated as she was more sensitive to things, and she had gas problems which made her cry and fuss, and was very hard to put to sleep or nap... but she outgrew it. Phew! But she is still a "dynamic" and alert and creative & bright little girl for which I am glad. LOL.
One thing that we used was Hyland's Colic tabs... my girl did not have colic, but had bad gas problems and this helps gas too. It calmed her and she slept better. Just rambling here, sorry.
The Book "Secrets of the baby Whisperer-how to calm,connect, and communicate with your baby" by Tracy Hogg is great. I used this and found it very helpful and I referred to it often with my first child.
Your girl (this is per my Pediatrician who said from 6 months they can be this way, and it is developmental based) also may be going through "separation anxiety"....object permanence kind of thing. It is a phase and they will mature out of it...
Here are some links:
http://www.drgreene.com/21_1183.html
http://www.wisegeek.com/what-is-object-permanence.htm
Maybe she is teething too? Sometimes babies can be clingier when they are uncomfortable or sick.
Is she getting enough intake/feeding? Sometimes if a baby is in a growth spurt...they are hungrier and want to feed more and more frequently. 6 months, 9 months, 12 months etc. is a growth spurt time.
Was your baby always this way, or or is this a sudden behavior change? Perhaps it is developmental.
Talk with your physician... I don't have any answers but I can totally relate as my first child was quite spirited as well.
take care,
~Susan
www.cafepress.com/littlegoogoo