I would say that assaulting a police officer probably qualifies as breach of the peace.
The Capitol Hill Police aren't going to play it that way, though. If they believed that what she was doing was a breach of the peace, they'd have arrested her at the time.
The Constitution protects Congresscritters from being arrested while going to or from Congress while it's in session, or from being arrested while in session. It is NOT complete immunity while Congress is in session, although it's been interpreted that way in the past for relatively minor offenses like speeding, etc. She's being arrested while NOT attending session, or traveling to/from session, but arrested for a crime that happened while she was traveling to/from session.