Dear Amy: My boyfriend and I have been dating for four months. Despite his affection and eagerness to see me frequently, I knew he was still seeing other people.

His birthday was about five or six weeks after we met — about 10 dates into our relationship. Instead of being with me, he went away for the weekend with some friends. When he came back, he wanted to start seeing me exclusively.

We moved in together a few weeks ago. This past weekend, I casually asked about his birthday weekend. Now that we are together, I was hoping he would tell me who he was with. He has been very open about his past relationships. But he refused to talk about it. He said no good could come from me knowing. I tried to explain to him that it was better for me to know the truth and move on than to wonder.

It is quite obvious that he went with another woman. Now I can’t seem to let it go. I’m hurt that he didn’t want to spend his birthday with me. I’m concerned that there was someone else that he cared enough about to spend his birthday with right before we became serious.

I love him and our relationship is otherwise healthy, so am I wrong to be hurt and questioning things? If so, how do I let it go?

— Ruminating in DC

Dear Ruminating: This relationship seems to have progressed at warp speed. If you had dated longer before cohabiting, this birthday date might not make you so bananas now.

The first thing you should do is to make sure your STD tests are up to date. It seems logical that he was with another woman — likely for a previously scheduled special weekend/birthday trip that turned into a last-hurrah/breakup weekend — possibly someone he met after you two started dating.

Over his birthday, you two were still in the non-exclusive dating phase. You don’t get to feel hurt. You do get to feel curious.

Your boyfriend seems to be in the driver’s seat of your relationship. If he doesn’t want to disclose or discuss this, and you don’t want to move out, I agree with him that you should find a way to move on.

Dear Amy: My husband and I are parents of an only child. We love to travel as a family. However, my husband and I realize that as our son grows older, he may occasionally want to invite a friend along.

We are comfortable with this idea, but it has brought up some questions about the proper financial etiquette in such a situation. If we invited a pal, how much (if anything) should we be asking the guest (or, more likely, his parents) to pay for?

We would feel uncomfortable asking the guest to help pay for shared costs (hotel, gas, groceries, etc.), but what about tickets to attractions, significant meals out or airfare? We would hate to offend anyone by getting this wrong, but have no barometer.

— Wanderlust

Dear Wanderlust: If you invite another child to vacation with you, that child is your guest and you should pay for the child. That includes transportation, housing, food, tickets for attractions, etc. The child should be expected only to bring along “pocket money,” for extras.

Many times, parents will respond to such generosity by offering to pick up some costs — or insisting on it. Then you can respond: “If you’d like to pay for Timmy’s airfare, that would be helpful. Otherwise, he is our guest.”

Grateful and thoughtful parents who can afford it will often give the child extra money to treat your family to an attraction or dinner, as an acknowledgment and thank-you. They might also find other ways to reciprocate, such as inviting your son along on one of their vacations.

But do not extend this invitation unless you are fully prepared to cheerfully pay for it.

Dear Amy: Regarding older people getting dogs, three days before turning 80, I adopted a 7-year-old dog. Best thing I ever did — for both of us.

That was over four years ago, and yes, I worry about what would happen if she outlived me, but both our lives have been enriched. I still work two days a week in the business office of a nursing home, and she comes to work with me, putting a smile on many faces, loved by everyone.

— Patricia

Dear Patricia: You’re my hero.