When your temperature is off, your body is telling you something's wrong. Either you have hypothyroidism and the rigors of your training unmasked it, or your calorie intake isn't enough to handle your mileage increase.
Measure your temperature for a week to see if it is, in fact, low. Body temperatures are normally lower in the morning and evening and higher in the afternoon. If your temperatures are low during the day when they're supposed to be at their highest, it's a sign there's a problem. See your doctor if it's regularly below 98.6°F.
Low body temperature is a classic symptom of low thyroid levels. Excessive training could disturb your endocrine system, which controls the release of hormones and has an effect on body functions. Your doctor can do tests to reveal if you have chemical and hormonal deficiencies.
Body temperature can also be affected if your body fat drops too low. Runners have higher than average calorie needs. When you bump up your mileage, you should increase your calorie intake as well. Without the proper fuel, you will not only lose needed body fat (and thus feel cold more often), but your body will start to break down, making you susceptible to illness and injuries.
Warren Scott, M.D., is a sports medicine physician and runner in Soquel, California, and member of RW's Advisory Board.