Am I sandboxed? If so, why??

For one of my domains that I own since Oct 2010, I decided to change up the site a bit about a month ago (since it was not really ranking even for the exact domain match). Anyway, so I changed it up, resubmitted my site in various places + directories using WebCEO, and soon after a few weeks I saw some results. It was getting some visitors, starting to rank for the desired keywords (and some other unexpected keywords!).

That was great, until couple days ago visitors dropped significantly (10x less). It was already small traffic to start of with, but now it's close to none. I checked the keywords, and I cannot see the site anywhere.

Has my site been sandboxed? I used this website: searchenginegenie.com
to check, and according to them, it is indeed sandboxed. I do not know how reliable or trustworthy they are, but just using it as a reference.

I'd really appreciate some feedback on this. Even if I was sandboxed, I do not understand why since all my content was unique and there was no blackhat done (yet!).

Thanks

Update: I still did get some traffic, like 10 visits, from "Search Engines" according to G. Analytics. Would this be possible if I was sandboxed??

There is no definite thumb rule for sandboxing and Google has never acknowledged it too. But to be frank, when a website is sandboxed you would still see it in Google but it would be almost near oblivion in terms of SERP rankings. It is different from a site getting banned (due to copyright or other reasons) when it would completely disappear from Google. But if it was anyways getting 10 visits a day then I doubt it was even sandboxed.