iOS will do it after periods here. Sometimes annoying as well, but usually useful.
Any chance you are confusing comma and period?
Do you notice any special behavior when you type double spaces?
Edit: (Haha, see what you mean in your post. What web browser are you using?)
Check Settings -> General -> Keyboard
Have you tried toggling Auto-Capitilization and Auto-Correction to see what happens?
How about the auto replace, anything odd in there?