No more space on empty SD card?

I have a Nikon D5000 and a 4GB SD card I use in it. I've been using the camera a lot recently for photos/video for my internship.

I'm not sure what happened or what I did but all of a sudden my SD card's capacity was reduced to around 300MB, so I was only able to record a small amount of video at a time until my card was full.

I have a MacBook Pro so I usually upload all the pictures to iPhoto but a couple times recently I just manually transferred photos to my desktop and manually deleted them off the SD card in Finder.

I'm not sure if that's what did this but today I uploaded photos to iPhoto and when I went to finder, it said that the capacity on my card had been reduced to around 80MB.

I'm incredibly confused as to what is happening and why this is happening. If anyone has any advice on how to maybe restore everything back onto my card or how to avoid this mishap in the future, I'd be really grateful! Thank you!

Added (1). Geoffrey: Thank you! That worked!

This is an easy fix, and if it happens again, you'll know what to do.

First, put your SD card into your MacBook Pro, and make sure all of your pictures are saved. Then do a Spotlight search for "disk utility". Open up disk utility, and click on your SD card, it should be on the left-hand side under "Macintosh HD" (or whatever you named your hard drive). Once you click on your SD card, there's an "Erase" tab next to partition, restore, etc. Click erase and it will wipe everything off your SD card. This is why you want to save all of your pictures/videos before doing this.

Read your camera manual again

You are supposed to periodically format the SD card in the camera
Yes, FORMAT IN CAMERA. Not Mac / PC
Not delete

Follow this golden rule & you will have much less headaches and troubles

Periodically dump the contents of the SD card into your Macbook & format the SD card in the camera