Oh, for goodness sakes.
Article I, section 2: No Person shall be a Representative who shall not have attained to the Age of twenty five Years,
and been seven Years a Citizen of the United States, and who shall not, when elected, be an Inhabitant of that State in which he shall be chosen.
Article I, section 3: No person shall be a Senator who shall not have attained to the Age of thirty Years,
and been nine Years a Citizen of the United States, and who shall not, when elected, be an Inhabitant of that State for which he shall be chosen.
Article II, section 1: No person except a natural born Citizen, or
a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President
These sections existed in that language at the time of the Constitution's adoption. Now, since we had no problem electing Senators and Representatives, there must have been people who were "Citizens of the United States" and thus qualified to serve in Congress.
So, yes, as soon as the Constitution was ratified, citizens of the various States were also deemed "Citizens of the United States".
In Federalist Paper No. 2, Hamilton specifically notes that this proposed Constitution would expressly establish what had been the unwritten practice during the Articles of Confederation: national citizenship:
To all general purposes we have uniformly been one people each individual citizen everywhere enjoying the same national rights, privileges, and protection. As a nation we have made peace and war; as a nation we have vanquished our common enemies; as a nation we have formed alliances, and made treaties, and entered into various compacts and conventions with foreign states.
The Fourteenth Amendment did not create a new class of citizenship.