Can I be given a DUI because a motorist behind me called 911 and said I was drunk, if the officer never saw me?

In Pennsylvania, the answer is yes.The true question is whether you can be successfully prosecuted. The other driver's observations can justify the officer's detention or stop of you. Unfurtunately, you admitted to the consumption of a controlled substance. Depending upon whether Clonopin is a schedule  I, II, III, or IV  Controlled substance, the officer vas differnet burdens of proof.  Having not seen you drive, the key question is whether the officer can prove your ability to drive safely was impaired. IN Pa., that burden is actually less than the DUI  ( alchohol ) burden of proving that you were incapable of safe driving. Interesting case! Even if you swerved once, tennis ball or not, that very well may not be enough to prove an impaired abilty to drive safely.

            In Utah, like most states, an informant can provide the information that the peace officer relies on to establish the necessary reasonable suspicion to justify a stop of a vehicle.

            The Fourth Amendment to the United States Constitution protects citizens from unlawful investigatory stops of their vehicles by peace officers.[1] Investigatory stops of vehicles are only permissible if the peace officer has a reasonable suspicion that the driver has been, is, or is about to be engaged in criminal activity. Reasonable suspicion is dependent upon both the content of information possessed by the peace officer and its degree of reliability. Both of these factors are taken into account by courts when evaluating whether the peace officer had reasonable suspicion to stop a vehicle.

            In Utah, like most states, the facts relied on by a peace officer to support reasonable suspicion to stop a vehicle may come from an informant.[2] The Utah Court of Appeals articulated a three-factor framework for analyzing whether an informant’s tip provides the necessary reasonable suspicion to support a peace officer’s stop of a vehicle. They are: (1) The reliability of the informant; (2) The detail of the information provided by the informant; and (3) The corroboration of the informant’s tip by the peace officer’s own observations.   Utah Courts evaluate these factors under the “totality of the circumstances” in determining whether the peace officer was justified in relying on the informant’s tip to establish the required reasonable suspicion to stop a vehicle.

However, just because the police officer lawfully pulled you over, does not mean that you will be convicted.  There are a lot of other factors that will come into play.  You should consult with a local DUI attorney. 

Hope this helps.