I finally found out the reason behind the sudden slow down of the traffic and sales of my webstore. I was so stupid not to remember to check it out again because this already happened before.
The sitemap url of my webstore on my Google Webmaster Tools account have an error. This is because I changed some of the product's pages url to the new custom url that my store provider created. I thought of changing it because the word of each products is more visible or readable on the URL.
I have requested the support to help me create a new sitemap url to upload it on my Google Account. Geeeee!!! After I resolved this issue, I will prepare myself for another batches of orders.