I think I know what happened.
On Sunday I recorded a video with my phone that ended up about 125 MB or so in size.
I run my Seafile server behind Cloudflare, and unbeknownst to me, Cloudflare has a 100MB file size upload limit. When the app reaches 100% upload on this file, Cloudflare tells it it failed, and the app starts over. It seems to try many, many times (not the 3 like the Android release notes state) before exhaustively giving up
The app tried to upload this file several times on Sunday (1.39GB data used), Monday morning (2+ GB used) and Monday evening (another 2+ GB used).
When troubleshooting a separate issue I discovered this problem that was being caused by Cloudflare. Disabling Cloudflare resulted in this file being uploaded without any issues.
I am going to say there's a solid chance this is my issue, and what chewed up all my bandwidth.
Too bad the Android app doesn't use chunked uploading!