How much time are you leaving between sessions? This pain should go away after you've been doing it for awhile and your back strengthens. You can also do squats, deadlifts, or hyperextensions to increase lower back strength (do you already do these or otherwise lift weights?)
You say you're doing 30 seconds slow/fast for 15 minutes, which is 30 intervals, but plans I've seen have people starting with 4-5 intervals.
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