How Involved Should You Be in Your Kids' Relationships?

Recently I wondered if it makes me a little stalker-ish if I text my son's girlfriend to see if everything is ok with her and/or with them? I mean I was legitimately worried about them - both. My husband had commented that said son was sulking and not conversing with him and when said son came upstairs he looked upset (which according to him I always think he looks that way). I spent 5 minutes begging him to tell me what's wrong and he kept saying, "nothing". Then I saw his girlfriend had put the sad face :( emoticon on her Facebook status (again...stalker-ish maybe??) and I got even more worried! So, what's an over protective worried mom to do? I immediately texted the girlfriend to see if everything was ok. It took her a while to respond but she did say everything was fine just a little drama. I left it at that but I felt like a real idiot that I even went there and I did apologize to her. But like I said, I was worried about them. They like each other so much and are so cute together that I worry what they'd go through if they ever broke up. I know realistically high school sweethearts don't stay together forever. And if they do break up - one of them will have a legitimate reason but I don't want anyone to hurt either. So, did I cross a line? How involved should you be in your kids' relationships?