Is my girl cheating on me?

my fiancee, Macca, is bi. recently she met another girl at a party and they become very close very fast. Macca goes into the bassement and shuts the door to talk to this girl on the phone and when she comes back upstairs sshes all giggly and smiley. shes even avoiding the subject of this girl in front of her 13 year old son, which is a red flag. also Macca is 27 and this other girl is 22. i dont think shes particulaly attracitvie either, Macca is like way out of her league. they were grinding on each other at the party. Macca has pix on her laptop. i cant tell if its a really good friend ship or if Macca might be cheating on me. this would brake my heart because i love Macca with all my heart and were supposed to get married next summer. if shes cheating i wont dump her but ill be heart broken. im afraid shes cheating because we cant have kids i was upset about it. she doesnt want any but i did and i think shes trying to get back at me cuz we had a big argument. either way shes incappable of having any. her son is adopted. anyways. if shes cheating with a chick i dont know how we'd explain it to her kid. can some one plz help?! i love her so so much, i dont know what to do... :(