Can any of you fitness pros out there give a kettlebell newbie some advice? My heaviest KB is 25#. Yesterday I used it for the Beginner Kettlebell #6 workout, which was a total of 120 swings (4 sets of 30). Today, my lower back is sore. I also felt fatigue in my lower back when I was doing the swings. I was really focusing on my form, not arching my back, hinging from my hips and sticking out my butt, etc. I felt the burn in my glutes. But no matter what, my lower back gets fatigued and is now sore. My question is--do you think it's a form issue, or am I just strengthening my back muscles? It doesn't really feel like my back is "thrown out", it feels like muscle soreness on each side. I don't want to injure myself but the 15# KB is just super easy for me to swing and I'd like to keep working my way up.