drop in heart rate

kate
I have 33 week old twins. My daughters heart rate dropped when I was holding her right after feeding and I felt so helpless.  she was on the monitor I don't know if I would have noticed otherwise or known what to do, I was also feeding My son so I was distracted but they say you should keep twins on the same schedule. They said she might have gotten too much milk too fast? They sort of jostled her and patted her back to get her hr up? It was scary. My question is, what do I look for and what do I do something like this happens?