Do you use TM to back up external drives?
The behaviour is exactly as if:
- Time Machine ran when the external was dismounted
- The external was remounted
- Time Machine ran again
- The total amount in scope of the backup up from the external is greater than the free space on the Time Machine drive
Major flaw in the rewrite of Time Machine for Yosemite is that in the above scenario, Time Machine ignores the fact that most if not all of the external is already backed-up.
In Mavericks, in the above situation, Time Machine would scan the disk and work out the precise requirements, which could take a long time. To speed things up, Apple now assume that every file has changed and just take total space used on disk.
If this is the case, there is little to do to recover - as you say all your old history is gone and easiest way to start using Time Machine is to erase the disk and start agin
See Yosemite and Time Machine for at least 9 other people with same problem.