I'm on my second day taking doryx (doxycycline) 150mg and I been feeling tired is that a side effect from the pill?

Doryx (doxycycline) Secondarily you can become tired from the drug which can affect your BUN and cause drowsiness. However, you shouldnt be affected so early in treatment. Check with your doctor for a thourough exam.
Not usually. Sun sensitivity or gut sensitivity but not usually fatigue.