This is something that was never explicitly said, but I hope it was implicitly understood (with these strips above), that Lillian had a period of depression that made her isolate herself to most, on the exception of her brother. Which is why she took his leaving a little hard (back in the first chapters, when he wanted to move in with Evelyne). He was her last human connection to the outside world.
The incident with Matt only added to the progress towards that isolation. It wasn't what necessarily caused it, but it deepened it, as Matt was a good friend she wanted to remain friends with.
Like most things in life, it's an accumulation of certain events and acts that can lead someone to build a fortress to try and feel better protected. Or lead someone to start opening the doors again :)
*This is a rerun of the finished webcomic Go Get a Roomie! You can find these two strips in the old archive >here<!