Sorry to bother, but I need help and I know it’s been a while since you asked that question, but I would like to know if you ever got to fix your problem. You said your snaps got blurry only when you posted them and i’ve been experiencing the same problem. If you were able to fix it, can you please tell me how you fixed it.
Snapchat is using compression techniques when you upload your video in order to :
Save your time, because the most compressed the video is, the less time it takes to upload ;
Save their money, because the lowest the size is, the less bandwidh they consume.
Unfortunately, the compression is lossy, which means that the video will have a lower quality and thus looks crapier after compression
There are two workarounds :
Try to compress the video yourself by reducing its duration or by using a software to reduce its definition to something you find "acceptable".
Try to upload you video over a WiFi connection rather than a 3G/4G connection ;