Bing's image shows the point all are missing. Quinarii were made with dies smaller than those used for denarii. Bing's wildwinds provided image shows a coin of similar diameter but the circle of dots fits easily on the flan while the OP coin cuts into the legends. I suspect the coins started out at about 18mm and has been worn, corroded or trimmed to it's current size. This is not a quinarius but a poor denarius.
I was looking at this one that sold as one and it's slightly bigger with no flange .just checking I know it's probably not .
Welcome to Coin Talk. Not an expert, but I agree with the others this is a denarius, on a typical crowded Severan flan. Some of my Septimius Severus denarii are light weight. Here is one that is under 2 grams - it doesn't scream "fake" to me, and it is rather porous, but I wish it were beefier. Septimius Severus Denarius (204 A.D.) Rome Mint SEVERVS PIVS AVG, laureate head right / INDVLGENTIA AVGG, IN CARTH in exergue, Dea Caelestis riding on lion, holding thunderbolt & scepter; water gushing from rocks left. RIC 266; RSC 222; BMC 335. (1.80 grams / 17 mm)