Assuming it's RIC IV Maximinus Thrax 13, 20 mm is the right diameter for a denarius, however as you pointed out, the weight at 1.9 g is extremely low. Most examples are between 2.4 and 3.4 grams. A quinarius would be half the weight of a denarius, but would have a smaller diameter, closer to 14 mm.
So yes, I would be suspicious of the coin. It might be a replica as you suggest. If you have it on hand, you could perhaps get a dealer to check the silver content.