Please help me find out what's wrong with me....

Hello I have a 9yr old daughter , 4yr old daughter and pregnan with our first son.. fir a while now I have noticed that I tend to push my oldest away. When we lay on the couch I cringe as she holds my leg. I love my baby so what is wrong with me. If feeling a complete separation from her. It makes me sick to my stomach because I know I'm the problem! I don't give her the attention that she needs and I see that but I still choose to turn the other cheek... she doesn't go without anything. She's very well taken care of. I just don't have that motherly bond with her anymore. It all started shortly after I found out I was pregnant with my 2nd. I don't know what happened since then... please help me... could it be PPD?