This is a very common  situation with new dentures, and some older dentures too.
Sometimes, when new dentures are made, if your tongue is wide, the sides of it will fall on top of the back molar teeth and you are biting the sides of your tongue, even when your not eating food.  It is a constant rubbing of the tongue on the inner cusps of the teeth.  The molars may be positioned to close to your tongue, they may also be positioned to low, allowing the tongue to fall on top of them, or they be simply to sharp.  No matter what this cause is it can be remedied by your practitioner at your request.  Do not let this irritation persist, because your tongue will become increasingly irritated and cause other problems, so get it attended to.
Many times, when the dentures are made, certain areas that are shaped to allow proper movements of your tongue need to be attended to and adjusted more after you have received your new dentures.  As you wear your new teeth, they become compacted ( mostly the lower denture ) onto your gums, meaning that if there is an area that isn’t properly relieved for this compaction, it will result in irritation, or even an abrasion.  Again, very simply remedied by your practitioner at your request, don’t let it go on if you notice this.
Once you receive new complete denture and/or partials, not always, but often they need to be altered or adjusted to allow for the soft tissues and muscles in your mouth to move freely and not cause you discomfort. As a patient you need to communicate with your practitioner any problems you are having so they can be remedied as quickly as possible.
I hope this answer helps, take care,