Are there any muscle relaxant medications that do not cause drowsiness or tummy upset? for back pain, thanks

I'm afraid not: Tizanidine is the least likely to cause drowsiness (50% of people taking it get sleepy). The trade-off is that it's less proven to help acute back pain than cyclobenzaprine which seems to make almost everyone drowsy. Baclofen is just as bad. I usually recommend cyclobenzaprine at the 5mg dose taken just at bedtime for those who find it overly sedating during the day. Usually short-term use only