You have the right to retrieve your stuff from her room, but not to just hang out in there. You can also choose to call the police if you like. How one handles a situation like this is really up to each individual to decide.
Yes, you can call the police if you choose.
As a lodger, you can also evict her on 30 days notice for any reason or no reason at all.
Alternatively, you might consider a 3-day Notice to Quit on the basis of "nuisance."
Kevin King, Principal- Essential Law Services. HTTP://ESSENTIALAW.COM, 415-562-6862. The information presented here is general in nature and is not intended and should not be construed as legal advice for a particular case. This post does not create any attorney-client relationship with the author of the question answered. For specific advice about your particular situation, consult with me or another qualified attorney off-site.