Research studies show an association between anxiety, stress and nervousness with diarrhea (Lan et al., 2014). There is a relationship between IBS (irritable bowel syndrome) presenting with diarrhea, or IBS-diarrhea, with anxiety, stress, and nervousness. People with IBS-diarrhea usually suffer from anxiety (Lee et la., 2009).
So it can be ascertained that nervousness can cause diarrhea. So if you have diarrhea from anxiety, then you can treat both with treatments for anxiety, such as anxiety medications (Lan et la., 2014). Alternatively, you can try other treatments for anxiety.
Efficacy of tandospirone in patients with irritable bowel syndrome-diarrhea and anxiety. Lan L, Chen YL, Zhang H, Jia BL, Chu YJ, Wang J, Tang SX, Xia GD. World J Gastroenterol. 2014 Aug 28;20(32):11422-8.
Irritable bowel syndrome is strongly associated with generalized anxiety disorder: a community study. Lee S, Wu J, Ma YL, Tsang A, Guo WJ, Sung J. Aliment Pharmacol Ther. 2009 Sep 15;30(6):643-51.