Twingy back

Good morning!

I was hoping you might be able to help me please. I've just started rowing
again after 2 years and I'm loving it...well I'm not loving the erg
sessions, but I'm gradualy getting used to them!
I used to have problems with my lower back getting all twingy on one side of
my spine or other and it's happening again...actually it happened even when
I wasn't rowing, like during my yoga class for example!
I initially put it down to not stretching properly, but I can get out of the
bed a bit funny and it goes all twingy too! Are there any exercises to
strengthen one's lower back at all...or should I just stay in bed :)

Thanks!

Lou