Depression, that is, "major" or "clinical" depression, is so prevalent that many mental-health authorities call it "the common cold of mental illness." Depression has a host of known risk factors: female gender, family history, distorted thinking patterns, medication side effects, adverse life events ( divorce, financial reverses, the death of loved ones), and chronic illnesses (diabetes, multiple sclerosis, heart disease, Alzheimer's, Parkinson's diseases, and hormonal disorders).
As we get older, we must choose between different options in life again and again. Once we decide on one option, the others often disappear forever. If later in life someone is unhappy with the choices they made, it is common for them to start thinking 'What if?' thoughts. One important question is: Is 'What if?' thinking harmless, or does it make people feel unhappy about the choices they made and their lives in general?
So why are you feeling bad? After allowing yourself to accept accolades from friends and family, you start wandering around the dark corners in your mind of insecurity and self-doubt. Wasn't the lasagna a bit soggy? Did people really have fun or just say so to be polite? Despite the outward success of the day, you've circled right back to feeling like a failure.
When you are able to calm down, you should reach out to your friend. Chances are, she is totally unaware of her behavior toward you. Because she doesn't handle stress well, she shrugs off your issues. Now, seeing her co-worker on a daily basis and observing whatever crisis she may have experienced has forced her to acknowledge someone else going through it.