700MB would be alot in a few days if not downloading music, films IF you were on your own PC.
However, if you are accessing your office PC via logmein.com, information must be sent everytime you change what you are looking at or what you are doing on the remote PC. This is data usage and will be included towards your download limit. I have no I idea how many MB this uses but obviously its alot.
Hence even though you may be only downloading few files, the connection to the remote PC is downloading as well....
As an FYI, 50MB limit for the month is very very very low, when you get this sorted definitely increase that. I have O2 mobile BB and my limit per month is 10GB (that over 200 times your limit) and its E20 a month.