Piwik shows traffic lost after change website to https

About 1 month ago I changed my website to https links then I found I got less and less traffic when I check via Piwik. However, the data in my Google Webmaster Tool seems to be the same with before. So here is my questions:

1. Why does the traffic lost happen?
2. What's the difference between the 2 tracking tools?
3. Which one should I believe?
4. Should I do anything to stop traffic lost (if there is any) or change back to http?

Thank you for your replies!

1. I don't really know the reason for the drop in traffic.
2. Piwik and GA, both works in similar manner, however, piwik is an open source and it gives you exact data on user's behavior before it gets reflected by GA, when compared piwik, GA usually takes days to give you any useful information.
3. You should believe Piwik.
4. Change back to http is not necessary, cause it is not secured. Whereas sort out the exact reason for traffic loss and take necessary actions to prevent it.