This can happen especially if at the time of the LP they managed to hit a nerve. It sounds like it could have brought on sciatica, which is one of the common things that happens with LP's and LP shunts. I think that when they feel for the spaces in your spine for the procedure and the combination of the needle being pushed in, can aggravate your sciatic nerve.
Do you know if you have a problem taking anti inflammatory's? Some people are allergic to them or aren't able to take them.
An anti inflammatory can help which you can buy over the counter, such as Ibuprofen or you could try the Volterol which has Diclofenac in it, which is also an anti inflammatory.
It's also effective to use a heat pad or hot water bottle on your lower back. If it continues or gets worse visit your GP.
Heidi