My boyfriend's mom wants to go shopping with me?

Me and my boyfriend have been going out for almost a year. He is 15 and I am 14. He's out of town right now visiting relatives with his dad. While I was talking to him on the phone, his mom sent me a text wondering if I wanted to go to the mall sometime to go shopping for his school clothes. (I just think it's a little strange and I think it would be a bit awkward because I'm really shy around her and I just find it really awkward hanging out with his mom without him being there.) So anyway I was like "Braddd your mom keeps asking me to go to shopping with her" Like in the joking kind of way. He took it the wrong way and called his mom and asked why she keeps bugging me to go to the mall with her. She took it the wrong way thinking that it was my opinion and that I said that she was bugging me. So now I don't know what to do because if I go to the mall with her it would be really awkward, but if I don't then it woul hurt her feelings and she might not like me anymore. I know this sounds stupid, but I'm seriously like stressed out about this right now. D: