One way to test for a low thyroid at home is the "Broda Barnes" thyroid test. You get a basal thermometer at the drug store, then, the night before the morning you take your temp, you shake the thermometer all the way down, place it on the night stand. In the morning, place the thermometer under your arm and lie still for 10 minutes, when you read it, the temp should be around 97.6 and up, if not then you probably DO have hypothyroidism. Blood work just is not accurate, it is only a "snapshot" of what is going on for that second, however, it is good to check just to make sure, especially if the thermometer test indicates the low thyroid.
First, check your thermometer. Make sure to take your body temperature in the same way at the same time of day using two thermometers if you want to be very confident in the validity of your measurements. Some thermometers aren't particularly accurate, and body temperature varies throughout the day. So take it at the same time of day, at least 15 min. after eating, drinking, or otherwise putting anything in your mouth. Don't rely on ear thermometers or forehead measurements as these can fluctuate significantly.
As for causes of low/decreasing body temperature, the most common include hypothyroidism and related endocrine dysfunction, problems in the brain involving internal temperature regulation, loss of body fat (i.e.: insulation) as a part of loss of body weight (but only if this leads to being very thin), and so on.
Unless the body temperature is simply a result of weight loss or something else you can alter, you're going to have to see a doctor. You cannot diagnose or treat hypothyroidism at home, nor deal with other endocrine problems or CNS issues.