Running + CrossFit

CFWU x 10 x 2
(At home, so a few modifications: "supermans" instead of full back extensions, push-ups instead of dips, and slow negative pull-ups)

Run to the gym - 27 minutes
(I got a little lost in a new section of campus, so it took me longer than I'd hoped. But now I know where the veterinary hospital is, as well as the football stadium!)

CrossFit WOD 081029:
Shoulder press 1-1-1-1-1 reps (55#)
Push press 3-3-3-3-3 reps (55#)
Push Jerk 5-5-5-5-5 reps (60#)

Run home again - 15 minutes
(A more direct route!)

Question: I really feel the lifts in my mid- and lower-back. Is that normal? I tried to keep my stomach pulled in tight, but I think my back tends to arch when I arms are extended overhead.