I'm so scared *updated

We took Bridget to her one month well-baby check on Tuesday. She's five and a half weeks and has not gained any weight. Her head is growing, she's an inch longer, but she's still at her birth weight. I'm so scared. The doctor called it "failure to thrive" which is the worst term ever as it makes it seem like something that can't be overcome. 

I feel like a terrible mommy. I feel like I've failed her. Or her little body is failing her.  And I don't know how to fix it. 

The doctor didn't give us any real answers.  He said it could be anything from not eating enough or not metabolizing calories to enlarged heart or lung problems.  So for the short term, we're supplementing with formula.  I've been pretty much exclusively breastfeeding to this point.  Now I BF and then we offer her a bottle afterward. She has been eating from it so I guess I haven't been providing enough to this point.  Hopefully this will help her get on track.  I'm so scared it's something more serious though. 

We had to take her to get blood drawn yesterday (my heart broke while she cried) and then she had to get chest x-rays done. I'm calling the doctor every few hours hoping that he'll have some update at least on the x-rays. 

Have any of you gone through this? I'm completely freaking out and fighting back tears every few minutes. I just want her to be healthy and happy. Is that too much to ask?

**updated**  The Dr called and everything came back normal in her x-rays and bloodwork.  Actually, he said something in her bloodwork was a little high but didn't say what.  He wasn't concerned.  I am of course. However, I chose not to ask what it was because then I would just spend the rest of the week looking up reasons for elevated whatever-it-was.  I'll be taking her in next Tuesday for a weight check (my idea, not his) to see if this supplementing is working. Please, please let it work.