Hi,
I'm curious if you all might help me identify what my cat is doing...

Over the past 2 weeks our cat, RED, has begun to do something that I
think is odd.  I left a pair of pajamas out and she kneads them with
her front paws -- that is not the odd part -- she also now kneads with
her back legs as well (something I have never seen cats do before) and
then she does this thing where it looks like she might poop, but
doesn't (she's never peed or pooped outside of her litter box).
She appears to be very happy when she is doing this, purring extremely
loudly! and she does it almost every day for a few minutes at a time.
(I left the pajamas out because she seems to enjoy it so much)

Is she leaving her scent?
She doesn't do this on anything else, just my pajamas.
What is she doing? Should I be worried?
I'm curious to hear what you all think.
Thanks so much!!

A little background history:  Red is anywhere from 1-2 years old.  We
adopted her as a stray 6 months ago.  She was a scared and tiny cat and
it has took her about 3 months before she started to "play" or enjoy
catnip.  She is spayed, has normal healthy cat checkups, she eats and
drinks regulary, there is plenty of pee and poop in the litter box, she
plays with gusto, and purrs like a lawnmower.

The only changing variable is her weight. Just under 6 lbs when she was
found, Red has been steadily gaining 1 lb a month, since we adopted
her. When she hit 11 lbs the vet said that was probably good, and I've
been cutting back her dry food since then. Hopefully soon it will just
be wet, but it's a slow process, she just likes to lick the wet food,
and not actually eat it all.