Your hamstrings attach on your Ischial Tuberosity (your "sit bones", like when you tell someone they have a boney butt if they sit in your lap). Your Erector Spinae muscles are a group of muscles that run up your spine. Part of this group is the Iliocostalis that originates along your Sacrum (tail bone) and Iliac crest (your hip bone).
I am assuming that when you stretch for your hamstrings you are letting your back round out. This would allow the lower part of your back to stretch as well. Adding a 60 pound pack while doing PT is a lot of change for your body. If the pain is mildly uncomfortable but just "sore" your pain could be a few things. If your pain is hindering your range of motion or sends sharp pains anywhere you should go see a doctor as soon as you can. If you don't normally feel a lot of pain you should go regardless because your body might not be able to sense how hurt it really is.
If it is a muscle imbalance I would be willing to bet your lower abdominal muscles are under-developed compared to your back and your hip flexors are tight. Any of the exercises here should do the job for the lower abs. As a side note, I did not notice them tell you in the videos but your hands should be on the hamstring side of your hips (not in your lower back). This help keep your lower back in the floor, relieving the stress place upon it. You can stretch your hip flexors by performing this stretch.
If you do not feel comfortable doing this on your own, ask a trainer or someone in your unit that is more familiar with having "perfect" form while doing exercise.