Time Machine problem....

I typically back up my files every month, sometimes more depending on what I change. Today I tried to do my usual backup and received an error stating there's not enough storage space on my external HD. the external HD says there's 28GB available and i know i haven't added 28gb worth of information to my system in 30 days.
so i decide to delete all my previous backups. i'm not the kind of person that goes back to find out what i had on my system two weeks ago so that's not a concern. the external HD now has 220GB free. i have 60GB worth of RAW photos on it as well.
my MBP has 102GB out of 320GB free.

so what's the deal? why can't i back up my system? it appears as though i do in fact have enough space. I had my logic card replaced my Apple last week. would that have anything to do with the problem?

while i planned on upgrading to a seagate 500GB hard drive, it would be nice to be able to back up everything right now. any tips would be appreciated!

Cheers,
M

 

VM? what's that?