I wouldn't worry about this too much. I think it's very normal to just feel like crying in the middle of something and for no apparent reason. Pregnancy takes a big toll on your physically and mentally (you probably already know that), but sometimes you don't remember how much of a strain it is and BOOM, your body just needs to let it out in a crying fit.
The fact that you don't have many friends other than online and are probably alone most of the time also contributes to this. I have the same problems, and I feel for you. You will probably have more little crying urges to handle as the pregnancy progresses, especially in the last trimester when your hormones start preparing for delivery and being un-pregnant.
I think crying it out every so often is a great tension reliever, which is probably what you need. Most people in normal (not pregnant) circumstances who are tense and worried sometimes don't even realize that they are under some strain. Keeping it in only builds up more tension, so get out the kleenex
If you want to talk to someone, I'm always around...I've been through a lot of crying and pregnancies and I know how you feel.....even tho I'm not technically "there" in the flesh, which sometimes is really what you need....a real hug.
Congrats on the pregnancy!