My gf is overreacting/jealous?

This past week I've had to ride the bus home because my car is in the shop, so I saw some old friends of mine who I hadn't seen in a while on the bus (she was my date to homecoming last semester) I was sitting in the seat all the way to the back and 1 of my friends she was like please let me sit there I want to look out that window so on and so on (its a 1 person seat) she's like well can I sit there with you I'm like nope she's like can I sit in your lap? Nope. You can sit in my lap, I'm just like if you want fine, so on the ride home she was like holdin me we just talked, cought up and stuff, she massaged me (I just got out of weight training...) and when I got off the bus, she yelled out the window her number and toldme to text her. There was NO kissing, and I wasn't flirting, nor can I control who does it with me. But the thing is that she did it in like a friendly way. So yesterday me and my gf were going to see a movie, had a goodnight planned and everything, so I texted her and was like "hey baby you about ready for tonight?" So she responds "nah idk, how bout u jus spend it wit ur b***h". so I'm just wtf are you talking about? And she's like I heard how u we're all up on that lil b**** on the bus, 4real I don't even care about tonite n she called me a liar and hole bunch of stuff and told me not to bother talking to her until I got my stuff straight. How do I explain to her that I wasn't really even doing anything, because I hate the feeling that I'm feeling right now... with her not wanting to talk to me and just the tension. I really want to beat the **** out of whoever told her the lie that I was all up on another girl anyway... too bad I have no idea who it is