My mom is incredibly mad at me, how can I get her to stop ignoring me?

My mom and I don't fight much because she usually doesn't even know what is going on so there is really no reason for her to be upset. Ive been best friends with this boy that I have known for about 4 years (he is 14) and we started dating (lol I know parents wont understand because im really young so don't even bother telling me that im too young because I seriously don't care). Anyways, ive been seeing him for about 9 months and I didn't tell my parents or anything because I just didn't wanna talk abut it. He comes over every day but we don't ever do anything like kissing or anything when we are at my house because I didn't want my parents to bring it up so we just do all that stuff somewhere else. He came over today and my mom was making lunch and my dad was at work (he is a total workaholic) so we went upstairs to the family room just to hang out. He sat on the couch and I sat on his lap because I thought my mom was in the kitchen so we would have some free time. We were just talking about..stuff. We were just a little distracted that's all but then my mom just came in and it was too late for me to get off of him so I just sat there and my mom was like "Lunch is ready." and my boyfriend was blushing and I was kinda embarred so we just got up and went downstairs. He stayed for lunch but we just felt kinda awkward so he went home. I was helping my mom clean up and I was hoping my mom wouldn't mention it but then she just came out with it like "Im not blind you know? Ive known for a few months, why didn't you tell me?". I replied by saying "I didn't think it was a big deal.". She said "Well I think I would wanna know that my 13 year old daughter is dating, I cant believe you didn't tell me." I just said "Chill out mom, its not like if im going to marry the dude.". And now she just wont talk to me she just ignores me, its not like if its a big deal. I don't even know why it made her so upset just because I didn't tell her. How can I get her to chill out and stop ignoring me and making me feel bad?