Girl relax!
Sometimes it's as simple as what you ate the day before, or if you work out in the afternoon or evenings, what you ate for breakfast or lunch, you might not have "fueled" your body as well. It will also happen to me if I don't get a full, uninterrupted nights sleep. It can be a "that time of the month" thing going on, or girl, it can be the alignment of Jupiter to Mars, but I'm sure you didn't lose strength.
Weight training is a funny thing. Even some stressful event in your life, or a stressful week at work can effect the amount of weight you lift from day to day.
So don't fret, keep fueling your body with good stuff, especially prior to a weight training workout, and I'm sure you'll be back to lifting what's normal for you.
BUT~if this isn't a clitch, but a pattern that continues over the next week or so, you may want to get a check up and just make sure all is well internally. But 9 times out of 10, it's just a "happens to us all" type of thing.
Remember: fuel in = power out!
I try to eat a small carb/protein snack prior to weight training. Even a fat free or low fat yogurt 45 min-hr. before your workout can really make a difference. I keep it at about 150-200 cal usually. But I've noticed it makes a huge difference when I'm lifting heavier weights.
Let me know how your next workout goes,
Donna