My Cat Had Her Kittens 3 Weeks Ago Now She Is Getting Really Skinny. She Is Well Taken Care Of. What Is Wrong With Her Or What Am I Not Doing?

My cat has been losing weight lately and i can't figure out why. I take very good care of her. She is like my kid. She is well loved, fed,groomed, etc. It is like she has lost her appetite and she needs to feed her kittens. She is not puking or diarrhea. Just getting thin very thin. I found her on the road one day and she became mine and hasn't left me. She is happy here has never ran away always comes home when she goes outside. What am I doing wrong?