As catherinecherub says the forum cannot give medical advice. However, you do need to speak to your doc as soon as possible.
Unusual feelings of tiredness or drowsiness as well as feeling mentally cloudy are an uncommon, but known, side effect of this med.
Janumet is a combination of metformin and sitagliptin so it is unusual if you are taking it on top of metformin you were previously taking. It can reduce blood glucose quite rapidly so keep measuring and beware of hypos.
Seriously though, please contact your doc ASAP. Your tiredness is probably just a side effect of changes in BG, but it is also an early sign of lactic acidosis, which can be fatal and is a known (but quite rare) side effect of this med.
