Is my cat unhappy ???? :s?

My cat gives me contrasting signs. Her tail is constantly down, and she walks around the house the whole time, she wont play and she's mostly sleeping. All those indicate she's a sad cat but she gets a lot of my time, love and attention, I usually pet her till both of my hands get soar, she eats really well, and she sleeps on her back, which i heard means she's happy, she kneads her paws which is a sign she's happy. But I am confused, u see, we got our cat just a couple of months ago, she adapted to her new environment quickly and we became best friends in no time, although about a month ago she escaped and a week ago we found her, she's pregnant or so i believe. Is my cat unhappy ? Is it because she misses her previous owner ? Or is it just because she's pregnant ?