The roommates girlfriend thing you're probably right, but your door does not have to be locked.
TN law only states that one must unlawfully and forcibly enter. A closed door requires you to turn the knob. That is using force to enter. If you don't live there are uninvited you are there unlawfully (see trespassing).
The Castle Doctrine very clearly gives you a large latitude in considering Intent, Ability, and Jeopardy when in your own home. They give you the first 2 as assumptions. You would only need to clarify why you felt in jeopardy, but I'm not going to wait around to have a chat with someone who has entered my home that I do not know.
This doesn't mean I'm going to blast away at anyone who steps in the door, but they will have 1 second to obey my verbal commands or it will be an unpleasant ending.