With an unusually light period
and history of unprotected sex, yes, possibility of pregnancy is always there. However, the urine test can be false positive or false negative. Hence do blood HCG estimation test. This is more confirmatory. If this is positive, you are pregnant. If this is negative, then the cause of light periods could be stress, PCOD, hypothyroidism or hormonal imbalance. Do discuss this with your doctor and get yourself examined. Take care!