Can cats give birth then have more kittens days later?

My cat had 3 kittens 60 hours ago. She is happy, feeding, going to toilet and nursing kittens brilliantly. After delivering the 3 I was sure I could feel another so took her to vets who administered an injection of oxytocin and advised he could "feel something" during internal. The injection did nothing and as she was ok I believed she must not have any more inside. Since then I have felt the kitten inside regularly. Is it possible that interrupted labour could last this long? I am taking her to the vets first thing tomorrow am worried she will need c section. I am a single mum working full time but money is yight, however, if she needs the op I will of course find cash somehow. What I want to know is what are the chances she may deliver naturally and will the kitten be alive?