Ground stone is difficult to give an opinion online with pics . In my opinion one side of the rock is flattened fairly well and does not look natural to me. I also see that the rock has been polished this can happen naturally but it looks like some areas are more polished then others. So if it is an artifact my bet would be grinding stone which was used in the pecking and grinding process. I am not an expert this is just my opinion. Here are a couple pics of a rock that I believe was flattened by use possibly a mano that I thought was interesting enough to take some pics of. I normally wouldn't post these pics because I don't think they would be excepted as artifacts in hand they look more compelling.
Its very difficult to determine if something like that is an artifact or not without taking into account where it was found, if other artifacts were found, etc. Also, you can't go by "it fits nicely in your hand" because any and every rock can be turned in such a way as to fit nicely in your hand. If you found it in a field with obvious signs of indian activity, I'd say it's possible.. if you found it among river cobble or something, I'd lean towards no because that type of wear can occur naturally. It's always hard with a find like that... I'd hold on to it until you can figure it out.
It's true- looking at a (possible artifact) can be tricky- or just plain impossible w/ location context. There are many things that can ware down a stone on one or more sides. From water-stream-sand erosion to glacial grind- and even the chemical properties of the soil. Some rocks ware down more easily than others, but rest assured- and I say this understanding that you are new here- it's a well-used line "it fits well in the hand" "It feels good- fits the fingers". Do not be taken by that. Smaller tool stones/ pecking stones would not be made to fit the hand. They would be made to do the job. No rock is 'worn' by a hand using it. Consider how long it takes for, say, marble steps in front of a court house or library to ware even just a little in the most trod-upon places.....
But keep looking! Don't use anything like pintarest or even google images to decide whether what you have is a tool/artifact or not. Go to verified sites and take a good look at what stone tools really look like- even when they're broken or not complete for any reason. We'll help you- but there are recommended sites here at T-Net, if you look around the site a bit Good luck and HH!