I would consult with both your pharmacist and doctor about it. The pharmacist should be an expert regarding med side effects so you might want to speak with him/her first. Then, talk to your doctor.
I've been taking Soma twice per day since May for severe spasms and cramps and I've found this seems to be the only muscle relaxer that really helps me, as I've tried several others with no success. And actually, I have the opposite issue as you....when I take Soma, I find my breathing to be much bettter than without. But of course meds affect everyone differently. As another poster suggested, you can actually cut the meds in half and see if that lessens your breathing problems. When I first started Soma, I took the 350mg dose at night, which was generic, and the 250mg dose in the morning, whihc only comes in brand name. But I have financial problems and spoke to my PM about it and he decided to give me the 350mg only to take both at night and in the morning. He told me if 350 in the monring makes me too wooozy, I can just cut it in half. But I've been taking it so long, I found no problems taking 350mg twice per day. For some reason, it seems to clear my lungs more and I can take deeper breaths, though I don't know why, as relaxers tend to slow things down. But my body is quite used to my dosing by now. If I were you, i would definitely discuss it with your doc and pharmacist.