hello bluebird,
If I hear you correctly, some pain initially came and went, but one has gotten worse? It may be nothing, but as Margaret mentioned, you can always consult a doctor if you are really concerned. If this came up with doing the exercise, I think that is it's probably nothing, but you may be doing something incorrectly.. I would look at two things:
1) your posture: Be sure your back is straight. You don't want a curve in that area where the pain is. If you butt is sticking out, and you are curving your lower spine, it can create alot of tension in the area of pain you mention. Especially if you are new to this, and perhaps don't exercise in general that much, etc. An hour is a long time, if you are in an incorrect position. Stand against a wall and check your posture. If you back is straight, the small of your back will touch the wall. You straighten the lower back by tucking the pelvis forward. Relax the hips, don't hold any strain after doing this, keep the shoulders down, relax.
2) If it is kidney related, in the sense of low energy, and you aren't replenishing or feeding yourself as you exercise(you mention breathing): Just practice sitting with a "straight" spine (straight, not stiff, pull or suspend the crownpoint, "align" the back to straightness). Practice breathing in this position. Place your left palm on your lower dan tien, right on top of left. Opposite if you are a women. Pull up gently on your privates to close off the bottom. Press your lips gently together pulling back to the corners of a "smile". This closes openings where chi can leak out. Focus on your breathing in this position. Breath universal energy into your lower dan tien. Think of all the energy coming in, how you are closed and storing it. You can also shift, and focus on the mingmen point ( point on spine opposite navel). With another focus, bring energy there as you breath , like small universe. You can also put your hands on your kidneys. Massage them and hold the palms there for a few minutes. Think of putting dark blue energy into the kidneys. You can use sword fingers from the front to clear the spine in the back. All the above focuses on breathing energy into the lower dan tien, and energizing the kidneys. "If" , you are taxing yourself, by doing the full set of level I, spending time doing these things would be a good warm up, or meditative/energizing approach. Then do a shorter set, say 20 minutes, and build up slowly. Famous words, " Force is followed by a loss of strength". Sometimes that force can be you pushing too hard.
3) sitting and doing the level I exercise where you hold the ball by the lower dan tien and breath energy into the lower dan tien as you spread the hands, and exhale releasing blockages back to the universe as your hands come together , would be a good one while you sit too, gathering and storing, releasing...
You need to track this pain, even if it is nothing. Vary your approach to get a fix on it. See what affects it. Length of time, posture, kidney energy, or just a passing blockage making itself known.
