How fast can a pregnant dog's temperature drop?

We just got back from out of town and today is her 60th day. She was very excited when we picked her up from a friend's house and was very active on the way home and when we arrived home. I took her temperature within 15 or 20 minutes of her running around and it was still at 100.2. I took it again 2 hours later and it was only 98.6. I couldn't tell if the thermometer was all the way in though because she was laying in an awkward position and she is already wary of the thermometer. Can a dog's activity cause the temperature to rise that much or can it drop that fast...or was it just a bad read?