You educate them to the best of your ability, educating them on the meds themselves, tell them why the meds were changed if you know, but obviously, don't guess as to why. PP had good points. I always inquire as to when the family last talked with the doctor, what they've been told (good to hear what the families know; sometimes it's a misunderstanding, and the family doesn't really understand what the doctor has told them). This is your opportunity to educate the families or reinforce what the doctor has already told them.
If the family tells you that they just talked to the doctor 2 hours ago, and there's not really an issue at hand, suggest they write all of their questions down, and schedule a time when the family can talk with the doctor. However, they do have a right to talk to a doctor, so if they say, "I want to talk to a doctor now" or are not amenable to talking with the doctor when he/she makes his next rounds, then you call the doctor. Always ask them "would you like to speak with the doctor now?" if they seem disgruntled, etc.
Doctors have to deal with difficult families sometimes, too. Part of the job. But, we can do our best to mitigate the situation first. This gets easier with experience. I had trouble at first, too....not really with not knowing what to say, but how to get the families off my back so I could take care of the patients. Sometimes, you have to say, "I'll be back to address your other questions, but I need to check on my other patient." Or you can address their concerns while you're caring for your patient. So, if you're in the room hanging an IV med or helping your pt up to the bathroom, talk with the family while you're doing those tasks. Will help with time mgmt, too, because really, the difficult part about difficult families is that they suck a lot of your time up, so you have to learn to be both helpful, but assertive, because you can't let them interfere with patient care.
Sounds like you did the right thing by calling. Just be glad you were going home lol. The night shift nurse probably was just unhappy because she has to deal with it now, but that's part of the job, and it shouldn't be taken out on you. You can't make overly needy people less needy lol.