Depression from unexpected places.

[laconicsax]

3. Try not to worry about it.

Something a therapist once told me was, "don't tell stories." It's way too easy to get caught up in a downward spiral by telling yourself stories about what could could happen.

It's really a vicious cycle--we tell ourselves that it could be something, and because our stupid monkey brains can't tell the difference between thoughts and reality, we react as though the hypothetical is real.
Before you know it, your whole mood has changed and you're telling yourself more stories about whatever, making your brain think it's really happening, and so on.

It's possible that your meds affect your thyroid. That information is available online (I've heard that drugs.com has an accurate database). The best thing you could probably do is read up on what the most likely possibilities are and ask your doctor. If there were a really serious problem, you'd probably know it by now.