Is it Normal? No.
Is it normal for me? Yes..
lol
Don't be too worried mate.
I'd say you probably didn't hold as tight for one rep or perhaps hyperextended a little at the top on a couple of reps.
When you get a chance, lay on your back and bring your knees up until your whole back (including lower back) is flat on the floor.
lay like that for 5 minutes so it all settles.
nights sleep and you'll be fine.
This is also the reason I don't like lots of reps.
you get tired and sloppy.
I've got one more week of 12 reps and then down to 7 weeks of 5 reps, then 2 weeks of 3s, then 1 week of doubles, then it's time to Beat Nick!
lol