i had that once, it was when i rode natural after riding only street for ages. it uses loads of different muscles bacuse you have to angle the bike downwards :)
i stopped riding for a bit and it cleared up, i saw the doctor, was of some use :P
if the pain continues then i would suggest you go and see a Mc. Timmoney chiropractor, really helped my back when i hurt it.(not for the same pain as youve described but it is good :( )
Will