Puppies born before day 59 usually are not developed and strong enough to make it. Since she is at 56 days, lets hope she isn't in labor just yet. There are some things that can throw off the temperature. One thing is not having the thermometer inserted far enough. It should be inserted about an inch or so. I use a digital so it beeps and lets me know but if you use one that does not beep, it should stay in for at least a full minute to accurately record the temperature.
The last week of the pregnancy, you should be able to easily feel pups moving around. You will see a clear or whitish discharge as well. The day before labor starts in earnest, you will frequently see a little loss of appetite and some vomiting of bile as they empty their stomach. The hormonal changes which trigger labor can cause nausea. Additionally, the dog's body tends to void the bowels as the first stage of labor start which is why I asked if she had small frequent bowel movements today.
I'm really hoping she is not in full blown labor or that she mated a bit earlier then you saw which might explain the symptoms. However, a drop to just 98.6 might well be just that the thermometer wasn't quite in far enough. My dog's temperature normally drops down to at least 98.2F. I'm going to give you a site that goes over this and whelping in great detail.
Here is an excellent site about whelping.
I would go ahead and monitor her closely tonight just in case it really was an accurate temperature. Otherwise, continue to take her temperature and look for the other symptoms as well. Be sure to have your vet's number handy.
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