I think my girlfriend is cheating. Need some advise please?

I been dating this girl for about 5 months. She always been very secretive about her friendship. She has introduced me to many of her female friends, and she tells me when she is going out with them. On the other hand she never talks about her male friends or mention their names, which I find it strange because she has lot of male friends, and they all are kind of her type, poets, writers, film makers, etc. For example there is this one guy she kept texting 2 days ago while we layed in bed. She was reading this long message of someone called Sean, when she notice me looking she quickly put her phone down like she was hiding something, after awhile she was on whats up app, and the name Sean was on top of the list recent texts. I was intrigue so I done a little research on social media i find out the their are friends on Facebook, Instagram, and that recently she been liking a lot of is posts. She invited me to watch a play written by some friend, I find out using social media that Sean is the friend she mention. Recently she is spending less time with me, its also been a while since we spend a full weekend together, and all this texting guys that she dosent tell me about, kind of make me thing something shady could be going on. I'm not the jealous type, but Im also not a full. I wish I could talk to her about this, but that won't be possible because she usually gets extremely defensive and aggressive, every time I question her secret life. Recently we been having much less sex, and usually says that she's too tired to do it, or she will stop in the middle of it, saying it feels uncomfortable. Please guys give me your honest opinion?

From a female perspective her defensive and aggressive behavior is how she "controls" you and the questioning- (ignore it wait for the ranting to stop) continue your questioning. Remind her that you're spending less time together, it's been a long time that we spent a full weekend and you've notice her texting other guys which is making you feel a little uneasy. All of these are RED SIGNS!! "Stop wishing you could talk to her" you're showing weakness- MAN UP!!PUT YOUR BIG BOY PANTS ON- Tell her that's the LAST TIME she will address you in an defensive and aggressive manner and mean it.. There is a saying " Find A Fool Bump It's Head" The Fool tells himself I'm not a fool, yet he keeps allowing his head to be bumped" She been taking your kindness, tolerance, consideration as weakness. You say "I WISH I could talk to her (passive), but that won't be possible because SHE won't allow it" (showing weakness). Catching her texting Sean while in bed with you, it was obvious she was hiding something. People who are secretive, sneaky have something TOO hide. Most people I know would have said " Who were you texting, Why is he texting you? What's going on? Let me see your phone? The relationship is 5 months old time to get some answers to your questions ask her if she's been seeing any other guys? Ask who is the Sean guy you notice her texting the other night? Ask why she never mention that it was Sean who wrote the play? Inform her you have A PROBLEM with her texting guys that she doesn't tell you about. You say you've been dating "this girl"(how old are you guys?) Possible reasons for:a) "she's too tired to do it" -her excuse not too. b)stopping in the middle to complain it feels uncomfortable- could be due to an STD or a female vaginal issue (which requires medical attention).