The Green eyed Monster

Jealousy can be defined by www.dictionary.com as:

jeal·ous·y
   /ˈdʒɛləsi/ Show Spelled[jel-uh-see] Show IPA
–noun,plural-ous·ies for 4.
1.
jealous resentment against a rival, a person enjoying success or advantage, etc., or against another's success or advantage itself.
2.
mental uneasiness from suspicion or fear of rivalry, unfaithfulness, etc., as in love or aims.
3.
vigilance in maintaining or guarding something.
4.
a jealous feeling, disposition, state, or mood.

Jealousy is even considered as one of the original seven deadly sins as defined centuries ago by the Roman Catholic Church. Joseph Epstein, in his book titled "Envy", defines it as: to be jealous is to want to protect what you have.

What I would like to know, is at what exact point does jealousy become a sin? Take for example the events that unfolded over the past day or two.

I live in a rather modern house with my fiance and her son. Out of a fortnight, we only get one day together. I work Monday to Friday, she works Saturdays, and then every second Sunday, she has to take her son to his fathers - leaving one Sunday out of the whole 14 days where we get to just relax and enjoy each others company. She was on the couch, curled up in a rather picturesque position under a blanket; her gorgeous dark brown hair catching every ray of sunlight poking through the wooden venetian blinds in our lounge room. I sat quietly on the couch next to her as we both stared vacantly at the TV. Spontaneously, she receives a text message. Now my full intention was to spend this once-in-a-fortnightly day together, so I told her, "If that is XXXX, tell him to fuck off. I want to spend this day with you uninterrupted." She then turned to me and so pointedly replied, "It isn't", and continued texting.

This of course infuriated me, and I retreated to the study to find some kind of outlet; hence my being here. What followed was an argument of war-like proportions, where I tried explaining to her where I was coming from, only to be shut down at every angle. I was told that she was talking to a very good friend from high school, and that I was being stupid. Then came the silent treatment. Then came the denial. And then, came the apology. This could all have been avoided if but one of two things had happened.
1. If she had of just said, "No honey, I was actually talking to XXX. But yes, I will now turn my phone off."
2. If I had not have let the green eyed monster take over my ability to be rational, and therefore kept my mouth shut.

So, I now put it to you, my readers, if you are existent. At what point does jealousy become a sin? Was I wrong to be jealous that someone else was consuming her time, away from me? Was I wrong to question her about who she was talking to? Should I have just sat there, silently in anger? Is there a limit to caring too much?

Any insight would be great.

Ithler.