Time Machine not enough space

After searching the internet for a couple of hours I've given up and I'll bother you all with a question.

I've been using Time Machine since around december when I got a 1TB external LaCie. I've partitioned 250gb of it to back up the 180gb OS X partition on my mbp (yes I know it's not a ton of space for backup). It's been working wonderfully for 6 months. Even saved me once, found a file I had accidentally deleted.

I've had absolutely no trouble until yesterday.

I plugged in my Time Machine drive after 3 or 4 days of running around with my mbp and doing some light work. A few minutes go by and Time Machine has calculated that 85gb (usually it's at most 1-2 gigs for a full day backup, no idea how it went up to 85) and that there are only 74gb available, so the backup fails.

That's odd, so I try again. Same thing. So I enter Time Machine and I find that only one backup date is available, June 10th. Apparently the last time it was plugged in. So I exit Time Machine and browse through the drive itself via Finder and find that, sure enough, there is only one backup folder in existence. It seems like Time Machine deleted all my previous backups in order to fit this massive 85gb 4 days backup.

So does this mean everything is really gone, and that's why it wants to backup 85gb (just over half of the used space on my drive)? I'm pretty confused as to why this would happen, and what it means.

Any other options other than wiping and starting over?

Thanks