I don't think there are natural substitutes for lomotil and levibid. Natural things will only bulk and thicken the stool, not slow your gut. But maybe that is all you need? But if you are on an antispasmodic, you likely have IPS. If you're lucky, pregnancy hormones might make that simmer down.
How about trying Imodium? It is safe during pregnancy.
http://www.ncbi.nlm.nih.gov/m/pubmed/10758415/
Jan