Daughter afraid of the 2nd story

My daughter, Lexi, is coming up on 23 months old.  She is currently diagnosed with only PDD and testing on the autism spectrum.  She is in DI and OT therapy.

Lexi seems to be developing a worsening fear of the second floor of our home.  It started out with a fear of crossing the doorways upstairs and has developed into a fear of even walking anywhere upstairs.  So, when I take her upstairs for anything, she needs to be held.  I attempted to sit on our bed while holding her and she nearly hyperventillated.  I immediately stood up but it was too late and had to return downstairs with her in order to calm her down.  The only time this not a problem is in her crib.  She's fine then.  Lexi used to be ok in the bathtub but this seems to be a problem as well now.

Thoughts?  Suggestions?  She is due for her checkup at the end of next month but should I call the pediatrician sooner?  This is all so new to us and I still have so much to learn.