Panda and Penguin, every SEOs best buddies... 
I work with a company called Orion Group LLC. They are a great firm for off page optimization. I myself am not a great link builder, mainly because I am not a good sales person. I technically know the aspects of it, but have never been great at implementing it. I would give them a call.
I would say that if you want to decipher between Panda and Penguin you will have to go down the list of Content Vs. Links to really lay it on the line. If you have not seen a manual penalty then you have a chance to get things in order before the next update.
For Penguin:
1. Do you have links that don't generate traffic or have really keyword dense anchor text? Anchor text that is so repetitive you are basically keyword stuffing your link profile?
2. Do you have links to Payday loan stores, tobacco sites, porn sites or any other website with a low online reputation?
3. Do you have a lot of links that return 404 errors?
4. Do you have enough reputable links to outweigh any negative link juice? For example, a link to a DA of 100 (Pinterest, Youtube) compared to 10 links to sites with a DA of 10.
5. Is your link profile diverse? Do you have links in content, footers,blogs, links to industry related sites, links that send social signals (FaceBook likes and shares, Google +1s, twitter retweets)?
Panda questions:
1. Is my content uniquely valuable wherever it can be?
2. Do I have any User Generated Content?
3. Do I have any ads above the fold?
4. Is there any engagement on the pages that I am targeting keywords with?
5. Is my site easy to navigate and understand? Am I conveying my message clearly?
6. Do I have a ton of links in my content, whether internal or external?
7. Is my overall user experience pleasant?
I hope that helps you try to narrow down the pesky P named animal that is causing you trouble. I am sure with a Big Commerce template your UX is doing well. I hope that you figure this out soon!
The only other thing I would look at is mobile search traffic to see if that is having an effect. As well as the overall seasonality of your business. Is it possible that this is just a slow time of year?