Yesterday during the legs and back exercises, I could feel my neck starting to get tense; this morning it felt worse. I think it was caused by two reasons: poor warm up, and I was trying to squeeze in a couple more pull-up reps. I didn't get a good warm-up because I was tending to my daughter, who seems to need a diaper change every time we start exercising. Squeezing in a couple extra reps caused my form to suffer and put the tension in my neck instead of my back. I did some looking around to see what I could do better, and found some good advice and demos.
For now, I am going to enjoy my recovery day tomorrow and use a few basic techniques for relieving neck pain, such as soaking in a hot bath and getting a massage.