Is she overreacting ?

So I’m a girl & I am bisexual recently I came out the closet & I now basically have my first almost girlfriend. We have been talking for a few weeks & slept together (she’s my first). Last night we went to a kickback party with some friends drinking & smoking & these were her friends this was my first time meeting them. Well I introduced myself as her girlfriend to one of the people & when someone else asked if we were together I said yes. She never said much but she never flat out said no. We haven’t established a relationship but we are close & since we slept with each other & neither of us really deals with other people I assumed we’re basically together. I have strong feelings for her & I do think she feels the same but when we got home she mentioned it bothered her that I introduced myself as her girlfriend without speaking to her first. She says she’s private I feel like maybe she’s embarrassed of me like it’s been a few weeks but it’s genuine with us. She gave me the cold shoulder all night & I still feel her upset. What do I do.