Frequency congestion and confusion on short final to Runway 1 at DCA result in a go around for a CRJ-200 flight crew.

Narrative

On arrival into DCA; we were given vectors from OJAAY for the Mt. Vernon Visual Runway 1. The Approach Controller seemed to be very busy. After several heading vectors; altitude and speed changes; we called the river in sight and were cleared for the Mt. Vernon Visual approach. The Approach Controller did not instruct us to switch to Tower. At approximately 6 miles out we tried to verify a frequency change with approach; but did not get a response. We switched; on our own; to Tower frequency at the Wilson Bridge. We tried to make contact with the Tower; however; we received no response. At less than a 2 mile final for Runway 1; the Tower Controller acknowledged us and said we had been cleared to circle to land Runway 33. This may have been the case; however; this clearance would have been issued before we were on frequency; thus we could not have acknowledged the clearance. An aircraft was in position and holding on Runway 1 at this time. It was now apparent we would have to go around. The Tower Controller instructed us to go around. We complied with ATC instructions; and returned to land on Runway 1 without incident. It seemed both the Approach Controller and Tower Controller were under very high work loads. The resultant frequency congestion made it impossible to get verification in a timely manner. We should have switched to Tower earlier; rather than trying to get the Approach Controller to verify the frequency change.