It's not unheard of that overdue animals kid normally. Whether it's delayed migration of the egg/delayed recognition of pregnancy or just a weird phenomenon, or a major health concern is very hard to say just by guessing.
HOWEVER, it is COMMON - even among the very intense breeders that accidents can happen and she may not be due for another 3 weeks or several days due to short cycling or being bred on the next heat despite what the breeder thinks. You'd have to assess the risk of that depending on the breeder's information. If they sold all the bucks after breeding than you can be more sure that her duedate is accurate than if there are bucks on the property at all, for example. If they share a fenceline, if they have young-ish kids or the disinterested spouse helping with chores (who may very helpfully have put away an escaped buck they noticed in the doe pen and didn't write it down), or if you question the recordkeeping at all, or if the bucks share a fenceline with the does - then you just can't say. It's usually not IMPOSSIBLE, even if it is super unlikely according to the breeder.
A 'cloudburst pregnancy' or hydrometra can cause an animal to appear 'super pregnant' and gain a LOT of weight when really there is nothing but fluid in a gigantic fluid distended uterus, and in that case a quick ultrasound would clarify.
Another option is a fetal monster or fetal death causing a failure of onset of parturition. Fetal stress causes an onset of labor as the babies signal the dam that gestation is over and it's getting a bit crowded and the food isn't that great anymore.
But if the fetus(es) are not normal or dead, there is no fetal signal to the dam and thus an extended gestation.
You have a few options, and which you choose depends on what you're comfortable with. You can do nothing, and see what happens - be aware with fetal monsters you may be dealing with a deformity that cannot pass vaginally and a C-section would be the best option. You can have her assessed by a veterinarian for pregnancy status now or later with a quick and usually inexpensive trans-abdominal ultrasound. You can induce labor and expect kids - ready or not - in 36-48hrs depending on the protocol and drugs you use.
Also, what signs are you going by to diagnose the pregnancy? Belly size alone is not accurate, as some animals 'look' pregnant every day of their life because they are ruminants and belly size is mostly determined by rumen fill.