Are you diabetic?
Are you on any medications / herbs?
Do you have any present or past back issues?
The most likely cause for this numbness is herniated disc in the lower back (pinched nerve in the back). You would need to stop your weight lifting routine for atleast 7-10 days or till this numbness goes away. Following back care in the activities of daily living would help in remitting this numbness early;
a) Avoid activities which causes jerk to back.
b) Rest intermittently
c) Avoid bending at 90 degrees
d) Pushing and pulling should be avoided till numbness subsides
f) Avoid prolonged sitting and standing
g) Avoid sitting or sleeping on floor
A supervised physical therapy program would help you. Chiropractor manipulation too may be tried. Following investigations would be required, if this persists for long;
1) X-ray of the lower back
2) MRI of the spine
3) Nerve conduction velocity (NCV) study
4) Electromyography (EMG)
5) blood glucose, glucose tolerance test and glycosylated hemoglobin
6) serum B12 level with methylmalonic acid (with or without homocysteine),
Please feel free for your follow up questions.
You are very welcome.
The neck problem may not be related to it. But your mid back to lower back pain is likely to be related to it. This usually goes away spontaneously with good back care and has an excellent prognosis.
It is pleasure and privilege assisting you.
I hope this was helpful to you. If it was, please remember to ACCEPT the answer so I can get compensated for my work. If you have any more questions, please feel free to ask. I am truly privileged to help you, and I want to make sure that you are satisfied with the answer. Good luck!