Tens machines for back pain are totally different to those for labour, different type of frequency, have been advised not to use my back pain one when pregnant or in labour.
Have had back pain for years due to a weakened disc which keeps prolapsing, main things I can think of are warm baths, sounds strange, but a massage using something like a rolling pin can ease twingey nerves. Gentle movements can help, something like Kelly's suggestion!
Hope you feel better soon.