Advice? Boyfriend had a baby with another women.

Okay so last week my boyfriend called me at work to tell me that he was pretty positive that a girl he was sleeping around with before me and him started dating, had a baby 2 months ago. This baby looks just like him as a baby. I've compared baby pictures and everything. Now this girl he was messing around with doesn't actually know who the father of the baby is, given the fact she was messing around with a few different guys at the time. I'm seriously in a mix of emotions, because I know my boyfriend is the man of my dreams. He does everything I could ever ask of him, without me actually having to ask. I know by no intentions did he mean for this to happen, although he didn't take any precautions for this NOT to happen.. But I feel betrayed in a way, and I can't seem to shake the feeling, even though I know I should! It's a Baby! I should be happy right?! I'm so depressed though.. Especially since we had been talking about getting married in the next year and starting a family of our own, not that we still can't do that, but I fear he'll want to push it back now.. I need advice on how I should be dealing with this.. Do I really have any right to be upset? How do I shake this awful feeling?