Dramatic changes in weather will affect tire pressure. Sudden cold can cause low pressure readings as the air inside the tire contracts. Hot weather temperatures can cause tire pressure to rise as the air inside the tire expands.
Driving your vehicle creates friction that causes the tires to heat up. This will increase the pressure inside the tire.
It is common for tires to lose pressure slowly, over time due to seepage through the tire valve, past the tire beads, or even through the tire wall. Older tires will tend to lose pressure more quickly than new tires, but sudden, unexpected low pressure indicates there is a leak.