Help me my friend is mad at me and i dunno why?

okay yesterday at gym at school we were all sitting in the bleachers in the gym and i laughed at something my other friend Tia said then my friend Shany just looked at me with this face like a fake smile or something and she was bein real quiet and i was like 'Shany r u mad at me?' and she was like 'Yes im mad at u and Raigan' (Raigan's my other friend) so i was like 'is it cuz we dint sit wif u at lunch?' cuz at lunch my friend was beating the table and the janitor said DONT BANG ON THE TABLE and took the table and put it away and i was all oookkkaaaaayyyy so we had to find another table but all the tables were filled up and me and my group of friends sat down at this crowded table and there wasent any room for 2 of my friends (Shany and one other girl) so i thot maybe that was why she was mad but she said "no" and i said "tell me! what did i do??" and she wouldnt tell me but i think maybe she's jealous of Tia cuz she's me and Raigan's new friend and i think she might be jealous of her i dunno what to do but Raigan said if she doesnt talk to us for this week then she isnt our friend anymore but i rly wana stay frnds with her wat do i do??