Even perfectly fitting dentures will feel strange at first and can take a few weeks to get used to. They are likely to feel strange in your mouth initially, and may feel too large and cause you to salivate more than usual. You may even have some gum soreness to begin with.
This is all natural, as you have something new in your mouth that will take time to get used to. Within a month or two, your dentures should feel normal and comfortable.
You are advised to remove your dentures for a period of time every day (at night if possible) to give your gums a rest.