Is this normal or should I seek help?

I am the 2nd of 4 and tomorrow the last of my siblings is getting married. All 3 of them have kids, and I am truly happy for each of them. I will be turning 30 this year, I do not have kids and am unmarried. Ever since my younger sister (the baby of the family who is 5 years yonger than me) told me she was prego with baby #1, I've had random emotional episodes when I'm alone. I'll begin to feel sad and a bit envious of all those around me who are getting married and/or starting/expanding thier families. Then I will cry a lot. Sometimes it only lasts a minute, but sometimes it goes until I cry myself to sleep.

Is this a normal reaction for someone in my situation, or is this some form of depression? Has anyone gone through something similar?