Women can have platonic relationships with men. If you're feeling insecure rather than suspecting infidelity, you would have to work on that. When you tell her that her behavior is upsetting to you, she may interpret that as 1) not trusting her 2) not allowing her some autonomy 3) while the two had been apart, she's changed and had become more independent even in her interactions w/ men.
Her disinterest in sex could relate to many things such as - hormonal changes, self esteem issues, depression, not feeling emotionally connected/safe. She may also feel embarrassed or not known what causes her low libido. This may be a sensitive subject and you can bring it up by asking her what sort of intimacy feels comfortable to her, if there are new things she would like to try, if she still is in love with you as her partner, what you can do on your end to make her feel better and more desirable, etc. You can remember to compliment her during the day, do something special as a couple i.e. get couple's massage, get her flowers, etc. Just anything she enjoys and you can spice up the mood. During the day you can give her hugs, kiss her, etc. so she does not feel that physical intimacy only takes place in the bedroom and leads to sexual intercourse.