Hurt back doing shoulder press

[Nozza]

Hi

I have a history of lower back problems which have progressed to upper back and neck over the last few years. I have also had shoulder problems and have been doing RC strengthening for them for 6 months.

Last night I was doing some single arm shoulder presses with a low/moderate weight (able to do 18 reps). In order to achieve a range of movement that doesn't irritate my shoulder I was pressing further back than normal - ie my vertical forearm would have been further behind my head than normal (transverally abducted?!). I was OK for the first set but in the second I had an electric sensation in my upper back and how the whole area is in pain, I can't turn my head properly and it hurts both at the spine and around the shoulder blade.

I was hoping to find out:

1. What am I likely to have done?
2. How best to treat it in short and long term
3. How I might avoid it again

Many thanks

N