Last I heard, it still had not been determined who shot her. What makes you think the Secret Service was involved? They don't typically protect Congress.
Secret Service protect high-profile Congressmen and Senators. For example: Former first-ladies, presidential candidates, and others as appropriate. Hillary Clinton, for example, had an entourage of secret service agents wherever she went, including in the Capitol. Same with Obama leading up to the election.
TLDR: It's likely there are secret service members in the Captiol at any given time.
