Did you /r 1d20 or did you /r 1d20! I saw a new player make this mistake the other day... The first one rolls a normal 20-sider The second one rolls an exploding 20-sider, where, if you get a nat-20, it rolls again and adds them. He was using the ingame dieroller tool, and had accidentally checked the exploding box. Perhaps something similar happened to you.