It's doubtful that you are the victim of negative SEO, as you would have seen a spike in links (that is what negative SEO is, they attach spammy links to your site).
Seeing some variation in the rankings is not uncommon...Especially with the recent updates there is going to be an adjustment period. The rank dropped this week. They could very well get back on track next week.
Another question. Did anything big happen in your niche recently? Maybe there was a big announcement, and that changed the SERP's. News stories can change the rankings significantly because of Google's interest in providing fresh content.
Also, if you notice in the rankings graph, you did have a previous drop in rankings, only to have it come back again. So don't panic. As long as you are doing the right things, you should be fine.