When a person is depressed, it is normal to have good and bad days. Meds are great but they don't solve everything. I still have low days and anxious times even though I take meds.
Shy is right - try to work out what made you feel this way and that may help you to combat it in the future.
You could talk to your doctor about the possibility of increasing your med to see if that helps. Don't be too hard on yourself, it sounds like you have been doing really well so please don't let one bad weekend spoil it.
Try to concentrate on the good days you have had and that they outway the bad times.