PDOException = your website's asking for a lot of information.
If your MySQL server isn't accessible from the net and is only contacted via localhost from your web server, then probably not. This is more of an indicator that your server is underpowered or MySQL is misconfigured and needs tuning to keep up with traffic being generated by your site.
To see if there's any attack on your website, you need to consult your server logs. Most people underestimate what kind of traffic their site generates when their customers, Google, Yahoo, Bing, Baidu and Yandex decide to scour their site all at once. Yes, it is a Denial of Service event but not an attack.
Your server's falling over because it can't handle the traffic. You need to read through your web server access logs to determine this, an analysis tool like Weblog Expert can help out in this process. Once you've determined the traffic sources, only then can you determine whether it's malicious or too much valid traffic on a server that can't keep up.