I have a pinched nerve on my lower back now i'm having numbness on my right leg and buttocks and pain in my lower back. should i go to the er?

Not necessarily: if only sensory symptoms and low back pain is not excruciating it can probably wait till a soon outpatient neuro consult/follow-up with an expedient mri L spine. it sounds like you had one already recently showing a pinched likely S1 nerve root. the need for emergency visit would be if you develop sudden leg weakness and loss of bowel and bladder control