Three most likely things are retained placenta, ketosis, hypocalcemia.
Start by taking it's temperature to determine if it has a fever. Normal is 101.5-103.5 fahrenheit. If fever, it could be an infection from retained placenta if the placenta didn't come out. Hopefully this video is helpful in determining if it's ketosis or hypocalcemia:
https://youtu.be/vjaZolhJZTA?si=YCOjVobv1i0tJ9wd