Drama?

Okay guys this is a LOONG story
In geo class we are starting CPT and for this one we are suppose to work with either partners or in a group. So me and my friend Holly, decided we wanted to be partners for the project because we find best that when we work with two people we work harder. But, our other friend Emily came up to us and asked us if she can work with us and we turned her down and told her we wanted it with just Holly and I. But two days later, our other friend Heidi came back from her vacation. (shes kinda controlling and a bit bossy and mouthy but we stand her) And she automatically thought that she was working with Holly and I, we were going to tell her that class that we were just planning to work with the two of us but she was all upset that class because her grandma is sick. So I decided that we should just let her be in our group and everything, but Holly didnt want that but also didnt want to crush her feeling that class. So we decided to put it off till today to tell Heidi. When we told her today for the first time she kinda took it, but after during class she kinda seemed all pissed off at me. When I asked Heidi if anything was wrong she said no and that she wasnt pissed at me, but then she turned to Holly who is kind of softer but closer with me and told Holly that she still wanted to work with Holly and I. But, after Holly and I talked, we decided that we still didnt want her to work with us.
So today after school I called her up to see if she was still pissed at me and all, and Heidi said she wasnt but it sounded like she was still holding something back. And I don’t want this to blow up on me more than it has already.
So I need to know what I should do.
Suggestions?
P.S
I dont understand why Heidi is putting all the blame on me when Holly was the one that brought it up in the first place that she didnt want to work with her. AND we also didnt want to work with Heidi because we didnt think that it would be fair to turn down Emily but not Heidi.