Well, it is normal, as many people do this.
There are several possible reasons.
First of all, do you suffer from loose bowels or constipation? Do you have piles? All these will make your painful when you have bowl movement.
Secondly, this may be sign of you being in a complete relaxed state. Some say that they also have runny nose when they poop.
That is normal for people with problem in bowl movement(so called as constipation). If we use a great strength in bowel movement,the part with holes such as eyes will try to release the gas of our body, and the strength may be transferred to a pressure called IOP which will press the tear gland to induce tears.
So,don't use too much strength in bowl movement for that sometime the pressure may convert to your brain to cause cerebral hemorrhage especially the old people should note it.
Hello, don't be nervous, it's really normal. I have a friend who had the same situation with you. His doctor said that people will tears when they move bowels , just because they are too hard to do it. There have tear glands in our nose both sides, your eyes will tearing when the pressure more than your tear glands can fit. So, next time, easy your mind a little bit and remember eat more vegetables and fruits!