Why will my preemie will only take a fast flowing nipple?

I had a 34 week old premature baby girl two weeks ago. She was in the NICU for a week. She is now two weeks old. While in the hospital they used a fast flowing nipple for her, and now I cant switch her! She will not take the medium or slow flow. She gets really tired out and falls asleep. But with the fast she drinks it so quickly, and get a tummy ache. She hasn't spit up yet though. Is it safe for to drink from the fast flow? Ive always read that small babies should use a slow flow one...