Doctors have a saying: "If you hear hoofs, think horses not zebras."
In other words, while there are any number of explanations, start by looking at the obvious stuff.
And the most obvious is the low gear limiting screw on your rear derailluer. Trying turning the screw counter clockwise a couple rotations.
If that doesn't work, check to make sure the cable is too tight. Next up, check to make sure the derailluer hanger isn't tweaked. Bet it'll be one of those three possibilities.