I'd give it a go to remove senna. Aside from any wheat in it, I'd consider it a potent natural herb, I think it's a likely conclusion it's getting into milk. Yes obviously I am highly qualified to speak about this- not! - but base this on personal experience. Just think of how your breast milk can smell of garlic or spices if you been eating them, or that in my case, DS always had gripes tummy when I ate too many tomatoes... It might not affect every child, but if your child's,symptoms match the affect of you taking too much senna- very runny poos, gripes tummy, I thinks it's logical to try a removal of it from your diet....
I have had long term IBS issues manifesting in bad constipation so senna, movicol, lactulose and others have been regular 'friends'
Lactulose has always been billed to me as a 'benign' type of medicine which will support your digestion in flow through the intestinal tract, but if your body doesn't need so much it is absorbed just pooed out.
Whereas senna is an acute, 'get you going' type of tablet - for example i had a bought where I literally hadn't gone for a few weeks, the doctor suggested a course of 7 days on 1 or 2 tablets depending on the speed of affect. After the build up of constipation was cleared, I was back on maintenance measures - diet, lots of fluid, fibrogel and lactulose. Then if that was going well for a couple of weeks, I generally drop the fibrogel and stay on the lactulose, combined with watching my diet, intake of fluids.
So, sorry my point is, if you feel the need to take it every day, you might need some difference guidance and support on maintenance of your condition, how to deal,with acute bits of constipation, so your reliance on the senna is lessened. I am pretty sure fibrogel as lactulose is in preg/ bf ing...