Lumbar pain, leg weakness, internal tremor and constant feeling of heartbeat causing shakiness and insomnia

Ok.  I am a 30 yr Female, I started off with possible rectal bleeding (maroon stool and low back pain - center o lower back or lumbar area I guess you would say)...and had a colonoscopy, negative.  I had a ct of abd and pelvis, negative.  My back pain since worsened, a burning quality to it and it comes in waves, as does a less intense pain in my thoracic area, burning also.  I have accompanied bilateral leg weakness and shakiness, particularly muscular it seems.  BTW I have dizziness since 08 and essential tremor, both of which have worsened. For the dizziness I was taking lexapro 10 mg and xanax .5 xr and this helped it. Flash back before the back pain started I tried weaning off both medications, the lexapro completely, and down to .25mg of xanax with the help of my PCP's protocol.  I had some withdrawal for about a month and a half before the lumbar pain started.  I have since been put back on xanax .5 xr because of my anxiety and this hasn't helped at all.  I also have been experiencing an internal tremor that seems to originate from my lower back and vibrates in my chest and back of my head as well. I can feel it sometimes sitting up and always laying down at night...I need ambien cr 12.5 to sleep...because it is so disturbing. I likewise have this feeling of my heartbeat in my head and it is very intense, but not necessarily irregular or tachycardic.  So I have had a thoracic, lumbar and brain MRI all of which were unremarkable/negative with the exception of an under-developed sinus membrane on my left side seen in the brain (I am told this has no affect on my symptomatology).  I also had am EMG and NCS both of which were negative...me and my docs were concerned about some kind of neurodegenerative, muscular or demyelinating disease...they have ruled these out with the tests...I also have had blood word including thyroid, negative.  Can anyone help me with this...My doctors suspect the pain in my back is benign and brought on some anxiety and panic....but I am not convinced of this.  What are the possibilities.  I also have some fasiculations throughout my body and involuntary muscle movements originating in my lumbar pain.  I should note I have not had any kind of mechanical or other type of injury preceding the back pain.....nor do I exercise.  When I try to stretch my legs or try meditation, any stress I put on my lower back and legs instantly fatigues them and makes them shake.  Please help me.  I laso had an echocardiogram and ekg to rule out chest pain issues.  I will probably have a leg ultrasound done to rule out DVT due to high D Dimer of 1000.  Lastly, what else should I consider ??? Please help me as I am in distress, underslept, in pain and weak and shaky and therefore unsettled all of the time.