Questions about Labour

So I had my son 5 months ago and since then have searched the internet (to no avail) for some information. I had back labour, it was horrific. I was in constant pain from my first contraction until I started pushing. This is why I'm confused, when it came to pushing I went completely numb from the chest down - I couldn't feel anything! My contractions slowed almost to a complete stop (luckily it only took 3 pushes) because of this (I assume). I had two lots of diamorphine during labour but they'd worn off before it came to pushing and they didn't effect me like this anyway. No other pain medication.

My first question is, has anyone else experienced this?

My second question is, why did this happen?

Thanks!