For five years, Deidre Walker, 32, had dreamed of her boyfriend proposing. But when he finally pulled a ring out of his pocket and said, "Will you marry me?" during a romantic gondola ride in Italy, she didn't say yes right away.

Instead she whispered, "Put that thing away," and half-seriously considered throwing herself overboard into the murky water. Ten hours (and multiple conversations with her would-be fiance later), Deidre finally said yes, but she still spent her entire engagement feeling anxious -- and wondering if she had made the right decision. Her self-doubt even made her question the relationship: Was she feeling that trepidation because she'd said yes to the wrong guy?

According to a recent study of happiness trends on Facebook, Deidre isn't alone. Research scientist Cameron Marlow analyzed millions of posts on the social networking site and discovered (among other things) that when women get engaged, they actually become less happy.

Marlow suggests this could be due to the stress of planning a wedding, but Allison Moir-Smith, author of "Emotionally Engaged, a Bride's Guide to Surviving the 'Happiest' Time of Her Life," posits another theory: "Engaged women are battling with the myth that's been built up in our culture and their minds that they should only be happy, when in fact there's a lot more going on," she says. "A lot of women feel grief -- the end of single life, changes in family relationships, the end of simple dating days, and perhaps feeling isolated from your girlfriends who aren't in the same life stage as you."

Sue Griepsma, now 33, understands this perfectly. "I had been hoping for a proposal for at least a year when my boyfriend of four years finally proposed, and was therefore shocked when he did that I felt a sadness and a confusing sense of loss," she says. "It felt like a goodbye to the crazy, single me and hello to a husband and a mortgage and kids and in-laws. I was even sad about giving up the last name I had had for 31 years."

Which brings engaged women to a very important question: With all of these emotions flying around, how do you know if what you're experiencing is normal -- or if you're taking the plunge with the wrong guy?

"Red-flag issues," says Sheryl Paul, counselor and author of "The Conscious Bride: Women Unveil Their True Feelings About Getting Hitched." "I can usually spot them within the first 15 minutes of a session. Some common ones: addiction, abuse (emotional, verbal, physical), trust issues that haven't been resolved, or irreconcilable differences regarding core values like religion, money, whether or not to have kids."

If you have a red-flag problem, then your fears are about more than just you -- and you need to reevaluate your relationship. But fortunately, "about 98 percent of the people who find me are in excellent relationships but are just having trouble trusting that they're with a good partner," says Paul.

Which happened to be the case for both Dierdre and Sue. They are both currently happily married to their respective beaus -- and wish someone had been around to tell them that their sadness during engagement was completely normal.

"I definitely freaked out a little during that first month, thinking that the sad feelings I was experiencing were my subconscious telling me this wasn't the guy for me," says Griepsma. "Thank God I didn't listen to those feelings -- we're expecting our first baby in August!"

Colleen Oakley is a freelance writer who dulled any feelings of sadness when she got engaged with copious amounts of wine.