Why does not toilet bowl not hold water after new sink installation?

After hooking up a new utility sink to existing roughed-in plumbing, the bowl of the toilet nearby will not hold water level. (The toilet in the other bathroom is fine.) I suspect the new additional drain is the issue but do not understand why or what to do to remedy it. The bowl fills but does not swirl and swirl normallly, and the water level starts to recede immediately, emptying to a minimal level in about 10 seconds.