Exclusively eating plant based food will mess up you natural biota. Plant fiber is indigestible, it will claw at your digestive tract as it passes through. The high amount of stool you continuously produce will slowly alter your body's response to the stool. These are some of the things that could lead to digestive problems.
I did a quick internet search:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5571937/
"Conclusions: This study suggests that a long term vegetarian diets could be associated with irritable bowel syndrome. Nevertheless, further studies are needed to confirm these results, and investigate the multiple aspects of the vegetarian diet, possibly related to the IBS."