Can stress and thinking too much cause digestive problems?

Absolutely. The digestive system is very sensitive to stress in many people. From top to bottom, symptoms can include nausea, lack of appetite, stomach pains, stomach gurgling, diarrhea or constipation. It's best to manage the stress itself; in the meantime you might try over-the-counter digestive aids like pepto-bismol. See a doctor if symptoms are severe or persist.
Symptoms. The mind and body are actually one -- not separate. So worry, stress, and overthinking can easily be accompanied by digestive problems because the gut and emotions are richly interwoven. The process of mental worry affects your entire system, not only your digestion. Consider other examples like tension headaches, shortness of breath, skin problems, and heart palpitations also.