Hi Saffy,
I sometimes get some stinging in my lower backwhen I turn up my stim and I'm told this isn't normal so I guess yours might not be either. I'd mention it to your Doctor and see what he/she thinks. It might just need some tweeking and sometimes it can take a few tweeking sessions to get it right. Everyone is different and this is just my experience.

Take care and I hope you get sorted soon, don't let it go on too long

Sinéad