There can be several reasons for a delayed period and spotting. While negative pregnancy tests suggest that you may not be pregnant, it's important to consider other factors such as hormonal imbalances, stress, changes in medication (like letrozole), or underlying health conditions. Light pink and brown spotting can sometimes occur due to implantation bleeding or hormonal fluctuations. If you're concerned about starting letrozole while potentially being pregnant, it's best to consult with your healthcare provider who can provide personalized advice based on your specific situation. They may recommend further testing or adjustments to your medication regimen. Keep track of any additional symptoms you experience and discuss them with your healthcare provider during your lab appointment for a more accurate diagnosis and appropriate treatment plan.