Ever consider tossing out the scale for a while? It's probably the best thing I did for my diet. The scale is only ONE measurement of our success. ONE. ONE stupid, fickle, unpredictable measurement.
What matters is that it sounds like you had a couple of really great successes this week -- you watched your portions, showed control and restraint in situations that would tempt the best of us. You totally rocked Valentine's Day! This is the sort of behavior that will get you where you want to go -- but on your body's schedule.
You have absolutely no control over what the scale says from morning to evening to morning again. You have control over the actions that lead to weight loss, and you're there already -- you're doing what it takes. Exercise and conscientious eating. The scale will follow suit, you just have to trust yourself in that. You have no idea how many millions of processes are going on inside you, and what may have triggered that 3lb gain.
Just be patient, you're really doing the right things!!! Trust yourself in that, put away the scale for a bit, and relax!