I broke to with my girlfriend about five months ago. Due to living situations she stayed living with me until she could save up to fully moved out. Over these months we've been dealing with it but came to a form of normalcy. 

There has been some flirting time to time since our relationship but I chaulked it up tonl us dating nearly three years. Recently she attempted to kiss me, but I pulled out of it. I didn't think going down there road would be a good idea as we broke up due to us not exactly clicking very well, though I suspect it was due to me being stubborn and not wanting to change from enjoying not being alone; I won't deny that fact. 

Even though we aren't together anymore, I avoided dating anyone or get close enough to that  because it was still a fresh breakup and to not put her in a place of uncomfort while she lived with me. 

The other day I found out she's dating one of her friends and ever since I felt a sinking feeling and generally upset about it, and I'm not sure exactly why that is. 

Am I the asshole for being upset about her seeing other people while she's living with me? Am I upset just because I keep seeing her everyday and things would be different if we didn't live with each other?