Viagra can cause an upset stomach and/or indigestion, but check with your prescriber. It can also have cardiac side effects, and indigestion and cardiac issues can often feel similar.
https://www.goodrx.com/viagra/viagra-side-effects
https://www.healthline.com/health/mens-health/effects-of-viagra-on-the-body
https://www.forhims.com/blog/a-complete-guide-viagra-sildenafil-side-effects