Alot of women get back ache in late pregnancy just due to all that extra weight out front! Make sure up don't slouch as that will make it worse other than that there isn't really much else to do about it. I got leg cramps during my pregnancy - the midwife said it had something to do with lack of calcium I think (god it seems so long ago!) have a chat to your doctor/midwife they should be able to suggest something that may help.
Good luck