Shaking arms and stiffening legs when only on back

[EmmaWhit]

Hello,

My 4th month old seems to do only shake her arms and stiffen her legs when she is put down on her back or sitting in a reclined position.  I am very worried for her.  She first started out by simple shakes of her arms and it seemed to happen when she gets excited.  So at first, we were not concerned.  But now it seems to happen more and more and not just when she is excited, but also when bored or frustrated.  We recently saw her pediatrician.  He too noticed she was shaking or trembling her arms.  He said it was Fejerman syndrome or benign myoclonus and that it totally normal and it is due to an immature nervous system.  Made sense and we were relieved.

But for the past week after that visit, we notice she has increased in its frequency.  As soon as you put her down on her back, she seems to constantly do it.  It is scaring us.  I also notice she is beginning to wriggle her fingers now during these episodes.  On top of all of this, she also stiffens her legs.  Again, it appears mostly when excited since her breathing actually quickens during an "episode".  The episode lasts for 10 seconds and repeats.  She stops when we hold her hand or distract her like when she is playing with her toys.  But if you leave her there not doing anything or cuddling or talking to her, she does it.  Sometimes at the end of an episode, she sighs or kicks legs with excitement as if trying to let out pent up steam or energy.   She does not do it when breastfeeding,sleeping, being held or tummy time as far as we can notice.

We thought tourettes, spasms, seizures, but it doesn't seem to fit.  We also wondered if it is complex motor stereotypie.  We are sick with worry and we do have an appt with the pediatric neurologist, but of course we are very concerned.  

Any thoughts will be greatly appreciated.  Thank you!!