I just started doing strong lifts 5x5 program and am paranoid about hurting myself. I am almost 300 lbs, and haven't done much exercise other than walking for the past long as I can remember. I was having some problems achieving the correct squat depth, but today I was able to get down low enough. I am still only squatting the bar as I want to make sure I get my form correct before adding weight. Today I felt fine while and during the squats, but when i later got to the barbell rows I felt a twinge in my lower back. I decided not to finish my sets, because I don't want to hurt myself, but I need to know a few things.
A. Is having some lower back pain expected when doing this kind of stuff? B. Is it possible the lower back pain was caused by barbell row form or is it more likely that it was from improper squat form? C. If my back is hurting how long should I wait to try again?