would my mom sleep with my ex?

Omg I don't even know where to fucking beginning but I'm so depressed and hurt right now. actually I'M FUCKING DISGUSTED!! let me start off with the basic quick and easy. I'm 20, successful, ( as in i have my own house etc) I'm doing pretty good ...but me and my fiancee broke up and I had my mom move in with us because she was getting a divorce with my dad. anyway after me and him broke up ( fiance) I moved out I got another house but my mom stayed there then eventually she moved in with me. my mom is 45 and he just turned 26... anyway.... my mom always liked him thought he was a good guy blah blah little did she know the truth though anyway....i just found out she went camping with just him !!!!! like...what the fuck right.? I talked to my closes friends and they think because they are both adults and ALONE something will happen. my mom has been divorce for river a year but she has been dating this guy but they haven't had sexbeen having sex and I know she's sexually frustrated. ( we're really close we talk about everything )...but I can't help but think something is going on between her and my ex fiance. I have done nothing but help my mom ..give her a home. support her. etc...do you think she would do this to me?. like they went ALONE anyway this is what my ex told me. am I crazy is this normal?

( just to clarify she isn't living with him anymore! she lives with me)