I don’t know why but I get 1 more MB every 3 days. Have you noticed something similar with your gmail account?
edit: Ok, I finally get it. Check gmail counter.
Check: How to free up space in Gmail
and How do I ask Google for more Gmail Disc Space?
What happens if I near my storage limit?
We’ll keep giving you more storage space as we’re able, but if you do come close to the limit, the quota indicator displayed at the bottom of your account will change from green to red. If you go over your storage limit, Gmail will hold your messages for a few days while you clear out some space. If you don’t delete anything within that allotted time, you won’t be able to receive new messages, and messages sent to your account will then be returned to sender.