hi sun_flower,
do you mind if I ask how long you've been on the Effexor? Because I had the same thing happen to me. I'd been on it probably a year or so (with steadily increasing dosages) and I had a period where i'd get these stretches of time where I was feeling absolutely fine. As time went on, I'd get those "flashes" of normalcy more and more often, and they'd last longer and longer each time. Over a few months the depression got under control, so that I really only have the occasional flare-up anymore. With therapy and a med boost around stressful times like exams I'm a lot more in control of my depression.
Anyway, that's just my experience, but this could be a good thing! I agree with jilliebean, definitely call your doctor and let them know what's going on. Good luck.