OK, we can sort this out for you quite easily.
Firstly: were you a MobileMe subscriber who migrated to iCloud? If you weren't, you need read no further and take no action.
If you were a MobileMe subsriber you received complimentary extra storage when you migrated, due to expire on September 30 2013.
If at any time since you migrated you purchased extra storage it will have cancelled the free storage and will auto renew so you need do nothing.
You can check the situation:
- Choose Apple () menu > System Preferences, then click the iCloud icon.
- Click the Manage button.
- Click Change Storage Plan.
- Click Downgrade Options.
- Enter your Apple ID password, then click Manage.
What date is given here? -

If it's the same as in the picture, you did not purchase additional storage and need to purchase your current plan now. Click you current plan to select it then click 'Done'.
It it's not the same, the you purchased extra storage at some point and need take no action - you will not lose any storage. You will have been getting emails because the automated system doesn't know you purchased extra storage.