Hello, I have gone through your query and understood the concern. Menstrual irregularities are possible with use of levothyroxine usage, though frequency is not reported. This could be a cause of your missed period if you have started the medication recently. Black color in stool can be due to altered blood or some undigested food material. A laboratory exam will help to know for sure. I do not think cucumber can cause this discoloration of the stool, because it gets digested. Please contact your consultant for further management. If you wish to confirm/exclude pregnancy, please go for the blood test. Hope this helps.