High doses of synthroid are not uncommon, but it may reflect a relative decrease in absorption of the medication - make sure you are taking it on an empty stomach at least 30 minutes before food or other meds. Until the TSH is optimized (usually 0.1-0.5 initially after cancer surgery), it will be difficult to sort out symptoms.
The large lymph node may well be a normal "reactive" lymph node -- it depends on ultrasound characteristics and thyroglobulin levels whether observation is appropriate. An FNA biopsy is an easy procedure that would help address if this is a problem node.