Social Withdrawal around Girls in College

Hi guys. This is my first post here (I think). I've just recently started my journey on quitting PMO for good. I'm ready for the changes, and I know that if I can get in a good place and stay there, that I can be a wonderful human being to myself and others. I have a hard time staying there however, so I'd like to ask for some advice.

I'm in college right now, having gone back at the ripe age of 26. I've made friends and I have good times sometimes, but today in class I withdrew socially. I find it hard to stay stable. In this class, I'm often the guy who is extroverted, talkative, and makes people smile and chuckle. And yet today, I was shutting down mentally. I couldn't wait for class to end and to escape back into the safety of my apartment. I don't understand where this comes from or why this happens. I'm with girls who I often talk to, but today I was afraid to be there. Any advice?

Another interesting tidbit, I have Bipolar 2 disorder. I believe I manage it quite well however. I am medicated and enjoy the stability the medicine brings and I exercise as much as I can and dabble in meditation.