Is she cheating on me?

She came home after being gone for 4 ish hours she said the line at the bank was very long but admitted to going out with her friend Lynne and specified that she wasn't with a guy. Later she tells me her friend Lynne invited some guys to hang out at the park and they were drinking. When she came back she wasn't drunk or tipsy but I found a mark on her back that looks like a bite mark I'll post the links soon... thoughts??