325xi-2003 heat guage problem

I am new t the this Forum and am hoping someone can help. I have a 2003 BMW32XI and in the last several days it has been so cold where I live -20 that you cant be outdoors for any long period of time. I started the car the other day and all was fine, when several minutes later I noticed that the gauge was all the way to the right in the red. I stopped the car checked under the hood and all seemed fine. When I restarted the car the gauge went back to normal. The next day I filled up with coolant but the same thing happened,went full to the right stopped the car restarted it and gauge went back to normal. Today the gauge was normal but am concerned that the problem will come back. Could this be a sensor issue or perhaps something more serious?