My DD1 was three years old when I had my second, and yes, she did seem so much bigger than I remembered her. I think just the contrast of having a newborn to compare her to made me realize just how big she had become. Before that, she had been growing little by little every day, and I still thought of her as a toddler. But three is a funny age. They grow out of being a toddler and become a little child instead. And like all stages, one blurs into the next until it is suddenly there, but you are not quite sure exactly when it happened.
I did not feel the kind of sadness you are experiencing, but I did feel the shock and sadness at how quickly it all happens. My third daughter is most likely my last. And that is very hard for me. I mostly hate being pregnant, but I LOVE the baby and toddler stages. It is heart breaking for me to imagine life without a baby in the family. I am enjoying every age with my children as they grow. And as they get older, there are definitely perks of independence, conversation, learning new things together, funny kid quotes, etc. etc. So there is a lot to look forward to after the toddler stage. I just really love babies and toddlers. lol
It is all a lot to deal with (having a new baby, realizing how quickly they grow, and acknowledging a final pregnancy). The crazy hormones make it so much worse. It is ok to have these feelings. But do talk with your husband about them. Ask him to watch you and make sure it doesn't spin out of control. I agree with PP advice to speak with someone if it hasn't improved in 2 weeks. But otherwise, just know that newborns bring with them all kinds of feelings. Newborns change your life forever, and it can be a lot to process. Hang in there, momma! And congratulations on your sweet little girl!