It probably happened because of some uncomfortable sleeping or sitting position, causing a strain in my lower back which I felt as of yesterday. Didn't think much of it at the time, but today when I bent over to pick up Momo (I hadn't even touched her yet), I felt something give and I had to spend some minutes flat out on the floor hoping to straighten out my spine again.
Been in pain the rest of the day. In-laws were really nice. May brought dinner over 'cos I didn't feel well enough to walk, and Ling brought over a Salonpas muscle relief patch to ease me through my sleep.
Doubt it's the kind of pain that will just go away in a few days. Will seek medical attention in the morning.