I take it that your hill work involved you having to ride out of the saddle for longer than usual, if that is the case then you have given some different muscles a longer than normal work out.
These muscles will tighten after use and even if you stretch (and you should) they will still be tight. Now if you are unlucky like me, the ones in the buttocks can pinch on the sciatica nerve (this nerve runs down the leg even under the foot) however this should go as the muscle relaxes, therefore you are better of relaxing the next day after your hill work, until the muscles adapt or get stronger to your workout.
If the pain does not go, then it is possible to have bulged a disc slightly (body rocking at the waist as you struggle up a hill) and this could just be pressing on a nerve in your lower back (but the pain can be felt elsewhere) this takes longer to go back, but there are specific exercises you can do to help.