I have GERD and severe IBS and I guess the two in combination are causing the nausea. The sudden weakness and nausea are both being attributed by my doctor to anxiety. (I do have an anxiety disorder as well which tends to manifest itself in physical symptoms). I am being treated for anxiety and insomnia (finally), so I am doing a little better.
Thanks again to all who have replied.
With the suddenly weakness and nausea, I still think you could on top of what u already been diagnosed with.. you may have POTS which is a type of dysautonomia. (Dysautonomia is dysregulation of the autonomic nervous system which controls various functions in the body including your bowels).
http://www.dinet.org/
POTS is VERY COMMON in me/cfs people, quite a normal coexisting condition for us to have. POTS can also itself cause the symptom of anxiety, it also can cause IBS. Doctors put far too much down to just anxiety.
Be aware that also food intollerances are also very common in us..they too can cause those symptoms including sudden weakness, anxiety, bowel issues etc.
I do suggest you look into this all further rather to try to make sure you dont have more underlaying coexisting with ME/CFS conditions, then just putting it all down to anxiety and IBS. By working out your underlaying conditions, you can be treated better.
(my anxiety attacks and IBS may of all be due to the POTS I have, my GERD turned out to be due to issues with my insulin.. I was treated for GERD for years before I found out another condition was causing it and since Ive treated that other condition, I dont get GERD any more). Try to find out the whys of the conditions you have...