Thanks for your reply.
Unfortunately, I have already got assesed by two different health professionals, and none of them could help me. One said it was sciatica, and gave me a set of 3 exercises which didn't do anything. The other one, a physiotherapist, says it's a hip problem, but also couldn't help with his different set of exercises.
About this nervous/low back problem that you mention, would that be something easily treated, like exercises to strenghen muscle ? Or is it more complicated to fix ?
Thanks.