parents caught me and my boyfriend having sex.

some and my boyfriend are both 15 and have been together for almost 5 months. My mom already knows Im having sex btw. So basically me and my boyfriend went to my room because i had to get ready. We started kissing and stuff and we both made the decision to have sex. Then maybe 10 minutes in my mom is standing in the doorway and she started yelling “what are you doing!” we didn’t hear her walk down the hallway or come in my room. She asked him to leave and before he left she said to him, “you’ve really disrespected my house” then she just sat there not saying a word. now she wants to ground me, not let him into my room, and not let me go to his house again. I understand it was scary and probably disappointing for her to see that but I dont get why it made her so mad, when she found out we were having sex she just started laughing. She would always say she was very fond of him and now shes acting like she doesn’t like him. Please help me I need advice Im freaked out