cat had kittens three days apart..

Okay my cat sophia had her kittens on sat she had 4 one died then this morning (mon.) I realized another placenta with the litter and looked again a nd there was another kitten. Seems healthy. I bought milk replacment just in case. Seems since the other kittens are older they are hogging the milk. She is still panting and breathing heavy. Could there be more kittens ?

Yes, there may be more.  How is it going?  If she is still panting, you must contact a vet... Even be ready to rush in.

Some aftercontractions is normal, but if she has a new, long fruitless labor, its not good.

The FIRST kitten may have several hours labor, but next kittens shall have just short labor. It if is long - alarming!

Good luck!