By all means, call Dexcom and tell them that you dislodged the first sensor -- they'll probably replace it (though they'll want to hear that it was located on your abdomen...) I've had a few failed sensors lately because of the heat and sweating - they get replaced.
For convenience, I usually insert new sensors in the evening; however, they do tend to be less accurate the first 12 hours or so -- especially on the low side. That first night, I do try and be sure to double-check lows with a fingerstick before correcting it, especially if I don't feel symptoms. I don't always feel lows, so I don't want to ignore the warnings, but I have had more false alarms on the low side the first night after inserting a new sensor.
The other time I have to be careful is when I'm dehydrated - after a lot of exercise or many mornings when I awake. I find that drinking some water and waiting 10-15 minutes avoids that problem -- again, the Dexcom usually errs on the low side in that case.