Has she developed less interest in sex all of a sudden without any tangible reason? Is she frequently coming up with excuses, like insisting she is on her periods, just to avoid any sexual activity? Chances are, she is looking somewhere else.
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You could also be having sex alright, but the whole activity may suddenly get awkward. There is another clue many guys overlook, and this is when their women suddenly begin asking for changes in lovemaking. Be warned if she is asking you both to try out new bedroom styles or positions. Chances are she is doing those with another guy and only transferring them to you.