Hi Ainsely, as other have suggested, you can never rule out alogrym changes contributing to changes in your organic search. Penguin & Panda are just two categories of many. It just so happens that Panda and Penguin effected a higher percentage of all queries and so we get a bit more information about them.
When you have a dramatic drop, some combination of three things happened:
1. Google changed it's algorithm (or their crawler) in a way that effected you.
2. You changed your site in a way that effected what google sees.
3. External ranking factors (backlinks) for your site changed.
As far as I know, Google doesn't send you a message via GWT that says you've been effected by Panda or Penguin, but they are getting much better (IMHO) at transparency to web owners and we're starting to get very useful messages via GWT. So it's very important that you monitor it.
In the case of the original poster, they felt didn't make any changes that could have triggered #2, and the drop didn't sync up with a major observed alo change. So you'd be inclined to rule out the most likely suspects.
The challenge is that business owners don't have time to act as full time SEOs. So even though Carol noticed the drop around 2/18, that may not be when it actually occurred. That's why a a tool like SEOMoz's campaign monitoring is so useful, so you can see dramatic changes in your profile.
